System and method for supporting subnet number aliasing in a high performance computing environment

ABSTRACT

System and method for supporting subnet number aliasing in a high performance computing environment. In accordance with an embodiment, a fabric member can be assigned, by a global fabric manager, an alias fabric local subnet number in order to keep a fabric running after a fabric reconfiguration. The alias fabric local subnet number can be assigned for a period of time, the period of time being static, configurable, or indefinite.

CLAIM OF PRIORITY AND CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. patent applicationentitled “SYSTEM AND METHOD FOR USING SUBNET PREFIX VALUES IN GLOBALROUTE HEADERS (GRH) FOR LINEAR FORWARDING TABLE (LFT) LOOKUP IN A HIGHPERFORMANCE COMPUTING ENVIRONMENT”, application Ser. No. 15/416,642,filed on Jan. 26, 2017, which claims the benefit of priority to U.S.Provisional Patent Application entitled “SYSTEM AND METHOD FOR USINGSUBNET PREFIX VALUES IN GLOBAL ROUTE HEADERS FOR LINEAR FORWARDING TABLELOOKUP IN A HIGH PERFORMANCE COMPUTING ENVIRONMENT”, Application No.62/288,101, filed on Jan. 28, 2016, which applications are incorporatedby reference in their entirety.

COPYRIGHT NOTICE

A portion of the disclosure of this patent document contains materialwhich is subject to copyright protection. The copyright owner has noobjection to the facsimile reproduction by anyone of the patent documentor the patent disclosure, as it appears in the Patent and TrademarkOffice patent file or records, but otherwise reserves all copyrightrights whatsoever.

FIELD

The embodiments herein are generally related to computer systems, andare particularly related to providing network switch functionality in anetwork environment.

BACKGROUND

As larger cloud computing architectures are introduced, the performanceand administrative bottlenecks associated with the traditional networkand storage have become a significant problem. There has been anincreased interest in using high performance lossless interconnects suchas InfiniBand (IB) technology as the foundation for a cloud computingfabric.

The 48K unicast local identification (LID) value space in a single IBsubnet represents a limitation for the size of the subnet in terms ofnumber of end nodes. This limit is of particular importance whenvirtualized host channel adapters (HCAs) provide multiple virtual HCAinstances where each such virtual HCA instance may be configured with anindependent LID for each virtual port.

The IB standard specification defines router nodes in order to allowmultiple subnets to be connected within the same IB fabric and supportpacket forwarding based on mapping of fabric wide 128 bit destinationglobal identifier (DGID) address values to 16 bit destination LID (DLID)address values for each intermediate subnet as well as for the finaltarget subnet.

However, mapping of 128 bit DGID values to 16 bit DLID values at IB wirespeed requires complex content addressable memory (CAM) based lookuphardware that has limited scalability in terms of the number ofindividual DGIDs that can be uniquely looked up (i.e. within reasonablecost/complexity constraints for the hardware implementation). Thisimplies that in order to scale to a large number of individualdestinations, the mapping of 128 bit DGID values to 16 bit DLID valuesmust be flexible and must be able to use a hierarchical scheme so that alarge number of individual DGID addresses can be mapped via a singlelookup entry.

However, while a hierarchical mapping structure represents scalabilityin terms of the total number of subnets and end nodes that can berepresented, it also represents a severe limitation of the ability toroute individual destinations independently in order to maintainload-balancing and QOS constraints for different flows and workloadsthroughout the multi-subnet fabric.

This is the general area that embodiments of the claimed invention areintended to address.

SUMMARY

Described herein are systems and methods for supporting subnet numberaliasing in a high performance computing environment. An exemplarymethod can provide, at one or more computers, including one or moremicroprocessors a network fabric. The network fabric can comprise afirst subnet, the first subnet comprising a first set of networkswitches, a second subnet, the second subnet comprising a second set ofnetwork switches, and a global fabric manager, the global fabric managerhaving administrative control within the first subnet and second subnet.The method can assign, by the global fabric manager, a first fabriclocal subnet number (FLSN) to the first subnet and a second FLSN to thesecond subnet, wherein the first FLSN and the second FLSN are different.The method can monitor, by the global fabric manager, the networkfabric. Based on the monitoring, the method can reconfigure the networkfabric. Upon reconfiguring the network fabric, the method can assign atleast one alias FLSN for a period of time.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 shows an illustration of an InfiniBand environment, in accordancewith an embodiment.

FIG. 2 shows an illustration of a partitioned cluster environment, inaccordance with an embodiment

FIG. 3 shows an illustration of a tree topology in a networkenvironment, in accordance with an embodiment.

FIG. 4 shows an exemplary shared port architecture, in accordance withan embodiment.

FIG. 5 shows an exemplary vSwitch architecture, in accordance with anembodiment.

FIG. 6 shows an exemplary vPort architecture, in accordance with anembodiment.

FIG. 7 shows an exemplary vSwitch architecture with prepopulated LIDs,in accordance with an embodiment.

FIG. 8 shows an exemplary vSwitch architecture with dynamic LIDassignment, in accordance with an embodiment.

FIG. 9 shows an exemplary vSwitch architecture with vSwitch with dynamicLID assignment and prepopulated LIDs, in accordance with an embodiment.

FIG. 10 shows an exemplary multi-subnet InfiniBand fabric, in accordancewith an embodiment.

FIG. 11 shows an illustration of a data packet format using InfiniBand(IB) addressing to access a packet forwarding logic in a networkenvironment, in accordance with an embodiment.

FIG. 12 shows an exemplary portion of a Linear Forwarding Table (LFT)for intra-and inter-subnet forwarding, in accordance with an embodiment.

FIG. 13 shows an illustration of forwarding domains in accordance withan embodiment.

FIG. 14 shows an illustration of modifying a packet header from anGRH/ISRN addressing mode format to a LRH/DLID based forwarding format,in accordance with an embodiment.

FIG. 15 is an illustration of a switch providing a mechanism defining aspecial switch port boundary for changing packet forwarding from anLRH/DLID addressing mode to a GRH/ISRN addressing mode, in accordancewith an embodiment.

FIG. 16 is a flowchart for a method for using packet headers for linearforwarding table lookup for both intra- and inter-subnet forwarding, inaccordance with an embodiment.

FIG. 17 shows an exemplary system for supporting subnet number aliasing,in accordance with an embodiment.

FIG. 18 shows an exemplary system for supporting subnet number aliasing,in accordance with an embodiment.

FIG. 19 shows an exemplary system for supporting subnet number aliasing,in accordance with an embodiment.

FIG. 20 shows an exemplary system for supporting subnet number aliasing,in accordance with an embodiment.

FIG. 21 illustrates a method for supporting subnet number aliasing in ahigh performance computing environment, in accordance with anembodiment.

DETAILED DESCRIPTION

The example embodiments are illustrated, by way of example and not byway of limitation, in the figures of the accompanying drawings in whichlike references indicate similar elements. It should be noted thatreferences to “an” or “one” or “some” embodiment(s) in this disclosureare not necessarily to the same embodiment, and such references mean atleast one. While specific implementations are discussed, it isunderstood that the specific implementations are provided forillustrative purposes only. A person skilled in the relevant art willrecognize that other components and configurations may be used withoutdeparting from the scope and spirit of the claimed invention.

Common reference numerals can be used to indicate like elementsthroughout the drawings and detailed description; therefore, referencenumerals used in a figure may or may not be referenced in the detaileddescription specific to such figure if the element is describedelsewhere.

Described herein are systems and methods for supporting subnet numberaliasing in a high performance computing environment in a highperformance computing environment.

The following description of the example embodiments uses an InfiniBand™(IB) network as an example for a high performance network. Throughoutthe following description, reference can be made to the InfiniBand™specification (also referred to variously as the InfiniBandspecification, IB specification, or the legacy IB specification). Suchreference is understood to refer to the InfiniBand® Trade AssociationArchitecture Specification, Volume 1, Version 1.3, released March, 2015,available at http://www.inifinibandta.org, which is herein incorporatedby reference in its entirety. It will be apparent to those skilled inthe art that other types of high performance networks can be usedwithout limitation. The following description also uses the fat-treetopology as an example for a fabric topology. It will be apparent tothose skilled in the art that other types of fabric topologies can beused without limitation.

To meet the demands of the cloud in the current era (e.g., Exascaleera), it is desirable for virtual machines to be able to utilize lowoverhead network communication paradigms such as Remote Direct MemoryAccess (RDMA). RDMA bypasses the OS stack and communicates directly withthe hardware, thus, pass-through technology like Single-Root I/OVirtualization (SR-IOV) network adapters can be used. In accordance withan embodiment, a virtual switch (vSwitch) SR-IOV architecture can beprovided for applicability in high performance lossless interconnectionnetworks. As network reconfiguration time is critical to makelive-migration a practical option, in addition to network architecture,a scalable and topology-agnostic dynamic reconfiguration mechanism canbe provided.

In accordance with an embodiment, and furthermore, routing strategiesfor virtualized environments using vSwitches can be provided, and anefficient routing algorithm for network topologies (e.g., Fat-Treetopologies) can be provided. The dynamic reconfiguration mechanism canbe further tuned to minimize imposed overhead in Fat-Trees.

In accordance with an embodiment, virtualization can be beneficial toefficient resource utilization and elastic resource allocation in cloudcomputing. Live migration makes it possible to optimize resource usageby moving virtual machines (VMs) between physical servers in anapplication transparent manner. Thus, virtualization can enableconsolidation, on-demand provisioning of resources, and elasticitythrough live migration.

InfiniBand™

InfiniBand™ (IB) is an open standard lossless network technologydeveloped by the InfiniBand™ Trade Association. The technology is basedon a serial point-to-point full-duplex interconnect that offers highthroughput and low latency communication, geared particularly towardshigh-performance computing (HPC) applications and datacenters.

The InfiniBand™ Architecture (IBA) supports a two-layer topologicaldivision. At the lower layer, IB networks are referred to as subnets,where a subnet can include a set of hosts interconnected using switchesand point-to-point links. At the higher level, an IB fabric constitutesone or more subnets, which can be interconnected using routers.

Within a subnet, hosts can be connected using switches andpoint-to-point links. Additionally, there can be a master managemententity, the subnet manager (SM), which resides on a designated device inthe subnet. The subnet manager is responsible for configuring,activating and maintaining the IB subnet. Additionally, the subnetmanager (SM) can be responsible for performing routing tablecalculations in an IB fabric. Here, for example, the routing of the IBnetwork aims at proper load balancing between all source and destinationpairs in the local subnet.

Through the subnet management interface, the subnet manager exchangescontrol packets, which are referred to as subnet management packets(SMPs), with subnet management agents (SMAs). The subnet managementagents reside on every IB subnet device. By using SMPs, the subnetmanager is able to discover the fabric, configure end nodes andswitches, and receive notifications from SMAs.

In accordance with an embodiment, intra-subnet routing in an IB networkcan be based on linear forwarding tables (LFTs) stored in the switches.The LFTs are calculated by the SM according to the routing mechanism inuse. In a subnet, Host Channel Adapter (HCA) ports on the end nodes andswitches are addressed using local identifiers (LIDs). Each entry in alinear forwarding table (LFT) consists of a destination LID (DLID) andan output port. Only one entry per LID in the table is supported. When apacket arrives at a switch, its output port is determined by looking upthe DLID in the forwarding table of the switch. The routing isdeterministic as packets take the same path in the network between agiven source-destination pair (LID pair).

Generally, all other subnet managers, excepting the master subnetmanager, act in standby mode for fault-tolerance. In a situation where amaster subnet manager fails, however, a new master subnet manager isnegotiated by the standby subnet managers. The master subnet manageralso performs periodic sweeps of the subnet to detect any topologychanges and reconfigures the network accordingly.

Furthermore, hosts and switches within a subnet can be addressed usinglocal identifiers (LIDs), and a single subnet can be limited to 49151unicast LIDs. Besides the LIDs, which are the local addresses that arevalid within a subnet, each IB device can have a 64-bit global uniqueidentifier (GUID). A GUID can be used to form a global identifier (GID),which is an IB layer three (L3) address.

The SM can calculate routing tables (i.e., the connections/routesbetween each pair of nodes within the subnet) at network initializationtime. Furthermore, the routing tables can be updated whenever thetopology changes, in order to ensure connectivity and optimalperformance. During normal operations, the SM can perform periodic lightsweeps of the network to check for topology changes. If a change isdiscovered during a light sweep or if a message (trap) signaling anetwork change is received by the SM, the SM can reconfigure the networkaccording to the discovered changes.

For example, the SM can reconfigure the network when the networktopology changes, such as when a link goes down, when a device is added,or when a link is removed. The reconfiguration steps can include thesteps performed during the network initialization. Furthermore, thereconfigurations can have a local scope that is limited to the subnets,in which the network changes occurred. Also, the segmenting of a largefabric with routers may limit the reconfiguration scope.

An example InfiniBand fabric is shown in FIG. 1, which shows anillustration of an InfiniBand environment 100, in accordance with anembodiment. In the example shown in FIG. 1, nodes A-E, 101-105, use theInfiniBand fabric, 120, to communicate, via the respective host channeladapters 111-115. In accordance with an embodiment, the various nodes,e.g., nodes A-E, 101-105, can be represented by various physicaldevices. In accordance with an embodiment, the various nodes, e.g.,nodes A-E, 101-105, can also be represented by various virtual devices,such as virtual machines.

Partitioning in InfiniBand

In accordance with an embodiment, IB networks can support partitioningas a security mechanism to provide for isolation of logical groups ofsystems sharing a network fabric. Each HCA port on a node in the fabriccan be a member of one or more partitions. Partition memberships aremanaged by a centralized partition manager, which can be part of the SM.The SM can configure partition membership information on each port as atable of 16-bit partition keys (P_Keys). The SM can also configureswitch and router ports with the partition enforcement tables containingP_Key information associated with the end-nodes that send or receivedata traffic through these ports. Additionally, in a general case,partition membership of a switch port can represent a union of allmembership indirectly associated with LIDs routed via the port in anegress (towards the link) direction.

In accordance with an embodiment, partitions are logical groups of portssuch that the members of a group can only communicate to other membersof the same logical group. At host channel adapters (HCAs) and switches,packets can be filtered using the partition membership information toenforce isolation. Packets with invalid partitioning information can bedropped as soon as the packets reaches an incoming port. In partitionedIB systems, partitions can be used to create tenant clusters. Withpartition enforcement in place, a node cannot communicate with othernodes that belong to a different tenant cluster. In this way, thesecurity of the system can be guaranteed even in the presence ofcompromised or malicious tenant nodes.

In accordance with an embodiment, for the communication between nodes,Queue Pairs (QPs) and End-to-End contexts (EECs) can be assigned to aparticular partition, except for the management Queue Pairs (QP0 andQP1). The P_Key information can then be added to every IB transportpacket sent. When a packet arrives at an HCA port or a switch, its P_Keyvalue can be validated against a table configured by the SM. If aninvalid P_Key value is found, the packet is discarded immediately. Inthis way, communication is allowed only between ports sharing apartition.

An example of IB partitions is shown in FIG. 2, which shows anillustration of a partitioned cluster environment, in accordance with anembodiment. In the example shown in FIG. 2, nodes A-E, 101-105, use theInfiniBand fabric, 120, to communicate, via the respective host channeladapters 111-115. The nodes A-E are arranged into partitions, namelypartition 1, 130, partition 2, 140, and partition 3, 150. Partition 1comprises node A 101 and node D 104. Partition 2 comprises node A 101,node B 102, and node C 103. Partition 3 comprises node C 103 and node E105. Because of the arrangement of the partitions, node D 104 and node E105 are not allowed to communicate as these nodes do not share apartition. Meanwhile, for example, node A 101 and node C 103 are allowedto communicate as these nodes are both members of partition 2, 140.

Virtual Machines in InfiniBand

During the last decade, the prospect of virtualized High PerformanceComputing (HPC) environments has improved considerably as CPU overheadhas been practically removed through hardware virtualization support;memory overhead has been significantly reduced by virtualizing theMemory Management Unit; storage overhead has been reduced by the use offast SAN storages or distributed networked file systems; and network I/Ooverhead has been reduced by the use of device passthrough techniqueslike Single Root Input/Output Virtualization (SR-IOV). It is nowpossible for clouds to accommodate virtual HPC (vHPC) clusters usinghigh performance interconnect solutions and deliver the necessaryperformance.

However, when coupled with lossless networks, such as InfiniBand (IB),certain cloud functionality, such as live migration of virtual machines(VMs), still remains an issue due to the complicated addressing androuting schemes used in these solutions. IB is an interconnectionnetwork technology offering high bandwidth and low latency, thus, isvery well suited for HPC and other communication intensive workloads.

The traditional approach for connecting IB devices to VMs is byutilizing SR-IOV with direct assignment. However, achieving livemigration of VMs assigned with IB Host Channel Adapters (HCAs) usingSR-IOV has proved to be challenging. Each IB connected node has threedifferent addresses: LID, GUID, and GID. When a live migration happens,one or more of these addresses change. Other nodes communicating withthe VM-in-migration can lose connectivity. When this happens, the lostconnection can be attempted to be renewed by locating the virtualmachine's new address to reconnect to by sending Subnet Administration(SA) path record queries to the IB Subnet Manager (SM).

Layers and Addressing in InfiniBand

The IB architecture is divided into multiple layers wherein each of themultiple layers operates separately and independently from the otherlayers. On one end of IB Layer abstraction, the IB Physical Layerdefines the electrical and mechanical characteristics of the IB system,and on the other end of IB Layer abstraction the IB Upper Layercommunicates transactions between host and remote clients. The IBTransport Layer operates to provide partitioning, channel multiplexing,transport services, and packet segmentation and reassembly when sendingand receiving data, respectively. Packet forwarding and switching withina subnet is handled at the IB Link Layer, and the IB Network Layerhandles routing of the packets from one subnet to another.

In general, the Network Layer defines the protocol for routing a packetwithin a single subnet and between different subnets. For this the IBarchitecture uses three different types of addresses. A first type of IBaddress is the 16 bits Local Identifier (LID). At least one unique LIDis assigned to each HCA port and each switch by the SM. The LIDs areused to route traffic within a subnet wherein Link Level switchingforwards packets from a device specified by a source LID (SLID) within aLocal Route Header (LRH) of the packet to a device specified by adestination LID (DLID) within the Local Route Header LRH. Since the LIDis 16 bits long, 65536 unique address combinations can be made, of whichonly 49151 (0x0001-0xBFFF) can be used as unicast addresses.Consequently, the number of available unicast addresses defines themaximum size of an IB subnet.

A second type of IB address is the 64 bits Global Unique Identifier(GUID) assigned by the manufacturer to each device (e.g. HCAs andswitches) and each HCA port. The SM may assign additional subnet uniqueGUIDs to an HCA port, which is useful when SR-IOV is used. Routersoperating at the Network Layer send packets containing a Global RouteHeader (GRH) between different subnets. The routers forward the packetsacross subnets using the unique GUID of each device. In the process, thelast router in the packet path towards the destination port in thedestination subnet modifies the LRH of the packets by replacing thesource LID in the LRH with the proper LID of the destination port.

A third type of address is the 128 bits Global Identifier (GID). The GIDis a valid IPv6 unicast address, and at least one is assigned to eachHCA port. The GID is formed by combining a globally unique 64 bitsprefix assigned by the fabric administrator, and the GUID address ofeach HCA port. GIDs are independent of LIDs and, as such, remainunaffected by subnet reconfiguration.

Fat-Tree (FTree) Topologies and Routing

In accordance with an embodiment, some of the IB based HPC systemsemploy a fat-tree topology to take advantage of the useful propertiesfat-trees offer. These properties include full bisection-bandwidth andinherent fault-tolerance due to the availability of multiple pathsbetween each source destination pair. The initial idea behind fat-treeswas to employ fatter links between nodes, with more available bandwidth,as the tree moves towards the roots of the topology. The fatter linkscan help to avoid congestion in the upper-level switches and thebisection-bandwidth is maintained.

FIG. 3 shows an illustration of a tree topology in a networkenvironment, in accordance with an embodiment. As shown in FIG. 3, oneor more end nodes 201-204 can be connected in a network fabric 200. Thenetwork fabric 200 can be based on a fat-tree topology, which includes aplurality of leaf switches 211-214, and multiple spine switches or rootswitches 231-234. Additionally, the network fabric 200 can include oneor more intermediate switches, such as switches 221-224.

Also as shown in FIG. 3, each of the end nodes 201-204 can be amulti-homed node, i.e., a single node that is connected to two or moreparts of the network fabric 200 through multiple ports. For example, thenode 201 can include the ports H1 and H2, the node 202 can include theports H3 and H4, the node 203 can include the ports H5 and H6, and thenode 204 can include the ports H7 and H8.

Additionally, each switch can have multiple switch ports. For example,the root switch 231 can have the switch ports 1-2, the root switch 232can have the switch ports 3-4, the root switch 233 can have the switchports 5-6, and the root switch 234 can have the switch ports 7-8.

In accordance with an embodiment, the fat-tree routing mechanism is oneof the most popular routing algorithm for IB based fat-tree topologies.The fat-tree routing mechanism is also implemented in the OFED (OpenFabric Enterprise Distribution—a standard software stack for buildingand deploying IB based applications) subnet manager, OpenSM.

The fat-tree routing mechanism aims to generate LFTs that evenly spreadshortest-path routes across the links in the network fabric. Themechanism traverses the fabric in the indexing order and assigns targetLIDs of the end nodes, and thus the corresponding routes, to each switchport. For the end nodes connected to the same leaf switch, the indexingorder can depend on the switch port to which the end node is connected(i.e., port numbering sequence). For each port, the mechanism canmaintain a port usage counter, and can use this port usage counter toselect a least-used port each time a new route is added.

In accordance with an embodiment, in a partitioned subnet, nodes thatare not members of a common partition are not allowed to communicate.Practically, this means that some of the routes assigned by the fat-treerouting algorithm are not used for the user traffic. The problem ariseswhen the fat tree routing mechanism generates LFTs for those routes thesame way it does for the other functional paths. This behavior canresult in degraded balancing on the links, as nodes are routed in theorder of indexing. As routing can be performed oblivious to thepartitions, fat-tree routed subnets, in general, provide poor isolationamong partitions.

In accordance with an embodiment, a Fat-Tree is a hierarchical networktopology that can scale with the available network resources. Moreover,Fat-Trees are easy to build using commodity switches placed on differentlevels of the hierarchy. Different variations of Fat-Trees are commonlyavailable, including k-ary-n-trees, Extended Generalized Fat-Trees(XGFTs), Parallel Ports Generalized Fat-Trees (PGFTs) and Real LifeFat-Trees (RLFTs).

A k-ary-n-tree is an n level Fat-Tree with k^(n) end nodes and n·k^(n−1)switches, each with 2k ports. Each switch has an equal number of up anddown connections in the tree. XGFT Fat-Tree extends k-ary-n-trees byallowing both different number of up and down connections for theswitches, and different number of connections at each level in the tree.The PGFT definition further broadens the XGFT topologies and permitsmultiple connections between switches. A large variety of topologies canbe defined using XGFTs and PGFTs. However, for practical purposes, RLFT,which is a restricted version of PGFT, is introduced to define Fat-Treescommonly found in today's HPC clusters. An RLFT uses the same port-countswitches at all levels in the Fat-Tree.

Input/Output (I/O) virtualization

In accordance with an embodiment, I/O Virtualization (IOV) can provideavailability of I/O by allowing virtual machines (VMs) to access theunderlying physical resources. The combination of storage traffic andinter-server communication impose an increased load that may overwhelmthe I/O resources of a single server, leading to backlogs and idleprocessors as they are waiting for data. With the increase in number ofI/O requests, IOV can provide availability; and can improve performance,scalability and flexibility of the (virtualized) I/O resources to matchthe level of performance seen in modern CPU virtualization.

In accordance with an embodiment, IOV is desired as it can allow sharingof I/O resources and provide protected access to the resources from theVMs. IOV decouples a logical device, which is exposed to a VM, from itsphysical implementation. Currently, there can be different types of IOVtechnologies, such as emulation, paravirtualization, direct assignment(DA), and single root-I/O virtualization (SR-IOV).

In accordance with an embodiment, one type of IOV technology is softwareemulation. Software emulation can allow for a decoupledfront-end/back-end software architecture. The front-end can be a devicedriver placed in the VM, communicating with the back-end implemented bya hypervisor to provide I/O access. The physical device sharing ratio ishigh and live migrations of VMs are possible with just a fewmilliseconds of network downtime. However, software emulation introducesadditional, undesired computational overhead.

In accordance with an embodiment, another type of IOV technology isdirect device assignment. Direct device assignment involves a couplingof I/O devices to VMs, with no device sharing between VMs. Directassignment, or device passthrough, provides near to native performancewith minimum overhead. The physical device bypasses the hypervisor andis directly attached to the VM. However, a downside of such directdevice assignment is limited scalability, as there is no sharing amongvirtual machines—one physical network card is coupled with one VM.

In accordance with an embodiment, Single Root IOV (SR-IOV) can allow aphysical device to appear through hardware virtualization as multipleindependent lightweight instances of the same device. These instancescan be assigned to VMs as passthrough devices, and accessed as VirtualFunctions (VFs). The hypervisor accesses the device through a unique(per device), fully featured Physical Function (PF). SR-IOV eases thescalability issue of pure direct assignment. However, a problempresented by SR-IOV is that it can impair VM migration. Among these IOVtechnologies, SR-IOV can extend the PCI Express (PCIe) specificationwith the means to allow direct access to a single physical device frommultiple VMs while maintaining near to native performance. Thus, SR-IOVcan provide good performance and scalability.

SR-IOV allows a PCIe device to expose multiple virtual devices that canbe shared between multiple guests by allocating one virtual device toeach guest. Each SR-IOV device has at least one physical function (PF)and one or more associated virtual functions (VF). A PF is a normal PCIefunction controlled by the virtual machine monitor (VMM), or hypervisor,whereas a VF is a light-weight PCIe function. Each VF has its own baseaddress (BAR) and is assigned with a unique requester ID that enablesI/O memory management unit (IOMMU) to differentiate between the trafficstreams to/from different VFs. The IOMMU also apply memory and interrupttranslations between the PF and the VFs.

Unfortunately, however, direct device assignment techniques pose abarrier for cloud providers in situations where transparent livemigration of virtual machines is desired for data center optimization.The essence of live migration is that the memory contents of a VM arecopied to a remote hypervisor. Then the VM is paused at the sourcehypervisor, and the VM's operation is resumed at the destination. Whenusing software emulation methods, the network interfaces are virtual sotheir internal states are stored into the memory and get copied as well.Thus the downtime could be brought down to a few milliseconds.

However, migration becomes more difficult when direct device assignmenttechniques, such as SR-IOV, are used. In such situations, a completeinternal state of the network interface cannot be copied as it is tiedto the hardware. The SR-IOV VFs assigned to a VM are instead detached,the live migration will run, and a new VF will be attached at thedestination. In the case of InfiniBand and SR-IOV, this process canintroduce downtime in the order of seconds. Moreover, in an SR-IOVshared port model the addresses of the VM will change after themigration, causing additional overhead in the SM and a negative impacton the performance of the underlying network fabric.

InfiniBand SR-IOV Architecture—Shared Port

There can be different types of SR-IOV models, e.g. a shared port model,a virtual switch model, and a virtual port model.

FIG. 4 shows an exemplary shared port architecture, in accordance withan embodiment. As depicted in the figure, a host 300 (e.g., a hostchannel adapter) can interact with a hypervisor 310, which can assignthe various virtual functions 330, 340, 350, to a number of virtualmachines. As well, the physical function can be handled by thehypervisor 310.

In accordance with an embodiment, when using a shared port architecture,such as that depicted in FIG. 4, the host, e.g., HCA, appears as asingle port in the network with a single shared LID and shared QueuePair (QP) space between the physical function 320 and the virtualfunctions 330, 350, 350. However, each function (i.e., physical functionand virtual functions) can have their own GID.

As shown in FIG. 4, in accordance with an embodiment, different GIDs canbe assigned to the virtual functions and the physical function, and thespecial queue pairs, QP0 and QP1 (i.e., special purpose queue pairs thatare used for InfiniBand management packets), are owned by the physicalfunction. These QPs are exposed to the VFs as well, but the VFs are notallowed to use QP0 (all SMPs coming from VFs towards QP0 are discarded),and QP1 can act as a proxy of the actual QP1 owned by the PF.

In accordance with an embodiment, the shared port architecture can allowfor highly scalable data centers that are not limited by the number ofVMs (which attach to the network by being assigned to the virtualfunctions), as the LID space is only consumed by physical machines andswitches in the network.

However, a shortcoming of the shared port architecture is the inabilityto provide transparent live migration, hindering the potential forflexible VM placement. As each LID is associated with a specifichypervisor, and shared among all VMs residing on the hypervisor, amigrating VM (i.e., a virtual machine migrating to a destinationhypervisor) has to have its LID changed to the LID of the destinationhypervisor. Furthermore, as a consequence of the restricted QP0 access,a subnet manager cannot run inside a VM.

InfiniBand SR-IOV Architecture Models—Virtual Switch (vSwitch)

FIG. 5 shows an exemplary vSwitch architecture, in accordance with anembodiment. As depicted in the figure, a host 400 (e.g., a host channeladapter) can interact with a hypervisor 410, which can assign thevarious virtual functions 430, 440, 450, to a number of virtualmachines. As well, the physical function can be handled by thehypervisor 410. A virtual switch 415 can also be handled by thehypervisor 401.

In accordance with an embodiment, in a vSwitch architecture each virtualfunction 430, 440, 450 is a complete virtual Host Channel Adapter(vHCA), meaning that the VM assigned to a VF is assigned a complete setof IB addresses (e.g., GID, GUID, LID) and a dedicated QP space in thehardware. For the rest of the network and the SM, the HCA 400 looks likea switch, via the virtual switch 415, with additional nodes connected toit. The hypervisor 410 can use the PF 420, and the VMs (attached to thevirtual functions) can use the VFs.

In accordance with an embodiment, a vSwitch architecture providestransparent virtualization. However, because each virtual function isassigned a unique LID, the number of available LIDs gets consumedrapidly. As well, with many LID addresses in use (i.e., one each foreach physical function and each virtual function), more communicationpaths have to be computed by the SM and more Subnet Management Packets(SMPs) have to be sent to the switches in order to update their LFTs.For example, the computation of the communication paths might takeseveral minutes in large networks. Because LID space is limited to 49151unicast LIDs, and as each VM (via a VF), physical node, and switchoccupies one LID each, the number of physical nodes and switches in thenetwork limits the number of active VMs, and vice versa.

InfiniBand SR-IOV Architecture Models—Virtual Port (vPort)

FIG. 6 shows an exemplary vPort concept, in accordance with anembodiment. As depicted in the figure, a host 300 (e.g., a host channeladapter) can interact with a hypervisor 410, which can assign thevarious virtual functions 330, 340, 350, to a number of virtualmachines. As well, the physical function can be handled by thehypervisor 310.

In accordance with an embodiment, the vPort concept is loosely definedin order to give freedom of implementation to vendors (e.g. thedefinition does not rule that the implementation has to be SRIOVspecific), and a goal of the vPort is to standardize the way VMs arehandled in subnets. With the vPort concept, both SR-IOV Shared-Port-likeand vSwitch-like architectures or a combination of both, that can bemore scalable in both the space and performance domains, can be defined.A vPort supports optional LIDs, and unlike the Shared-Port, the SM isaware of all the vPorts available in a subnet even if a vPort is notusing a dedicated LID.

InfiniBand SR-IOV Architecture Models—vSwitch with Prepopulated LIDs

In accordance with an embodiment, the present disclosure provides asystem and method for providing a vSwitch architecture with prepopulatedLIDs.

FIG. 7 shows an exemplary vSwitch architecture with prepopulated LIDs,in accordance with an embodiment. As depicted in the figure, a number ofswitches 501-504 can provide communication within the network switchedenvironment 600 (e.g., an IB subnet) between members of a fabric, suchas an InfiniBand fabric. The fabric can include a number of hardwaredevices, such as host channel adapters 510, 520, 530. Each of the hostchannel adapters 510, 520, 530, can in turn interact with a hypervisor511, 521, and 531, respectively. Each hypervisor can, in turn, inconjunction with the host channel adapter it interacts with, setup andassign a number of virtual functions 514, 515, 516, 524, 525, 526, 534,535, 536, to a number of virtual machines. For example, virtual machine1 550 can be assigned by the hypervisor 511 to virtual function 1 514.Hypervisor 511 can additionally assign virtual machine 2 551 to virtualfunction 2 515, and virtual machine 3 552 to virtual function 3 516.Hypervisor 531 can, in turn, assign virtual machine 4 553 to virtualfunction 1 534. The hypervisors can access the host channel adaptersthrough a fully featured physical function 513, 523, 533, on each of thehost channel adapters.

In accordance with an embodiment, each of the switches 501-504 cancomprise a number of ports (not shown), which are used in setting alinear forwarding table in order to direct traffic within the networkswitched environment 600.

In accordance with an embodiment, the virtual switches 512, 522, and532, can be handled by their respective hypervisors 511, 521, 531. Insuch a vSwitch architecture each virtual function is a complete virtualHost Channel Adapter (vHCA), meaning that the VM assigned to a VF isassigned a complete set of IB addresses (e.g., GID, GUID, LID) and adedicated QP space in the hardware. For the rest of the network and theSM (not shown), the HCAs 510, 520, and 530 look like a switch, via thevirtual switches, with additional nodes connected to them.

In accordance with an embodiment, the present disclosure provides asystem and method for providing a vSwitch architecture with prepopulatedLIDs. Referring to FIG. 7, the LIDs are prepopulated to the variousphysical functions 513, 523, 533, as well as the virtual functions514-516, 524-526, 534-536 (even those virtual functions not currentlyassociated with an active virtual machine). For example, physicalfunction 513 is prepopulated with LID 1, while virtual function 1 534 isprepopulated with LID 10. The LIDs are prepopulated in an SR-IOVvSwitch-enabled subnet when the network is booted. Even when not all ofthe VFs are occupied by VMs in the network, the populated VFs areassigned with a LID as shown in FIG. 7.

In accordance with an embodiment, much like physical host channeladapters can have more than one port (two ports are common forredundancy), virtual HCAs can also be represented with two ports and beconnected via one, two or more virtual switches to the external IBsubnet.

In accordance with an embodiment, in a vSwitch architecture withprepopulated LIDs, each hypervisor can consume one LID for itselfthrough the PF and one more LID for each additional VF. The sum of allthe VFs available in all hypervisors in an IB subnet, gives the maximumamount of VMs that are allowed to run in the subnet. For example, in anIB subnet with 16 virtual functions per hypervisor in the subnet, theneach hypervisor consumes 17 LIDs (one LID for each of the 16 virtualfunctions plus one LID for the physical function) in the subnet. In suchan IB subnet, the theoretical hypervisor limit for a single subnet isruled by the number of available unicast LIDs and is: 2891 (49151available LIDs divided by 17 LIDs per hypervisor), and the total numberof VMs (i.e., the limit) is 46256 (2891 hypervisors times 16 VFs perhypervisor). (In actuality, these numbers are actually smaller sinceeach switch, router, or dedicated SM node in the IB subnet consumes aLID as well). Note that the vSwitch does not need to occupy anadditional LID as it can share the LID with the PF

In accordance with an embodiment, in a vSwitch architecture withprepopulated LIDs, communication paths are computed for all the LIDs thefirst time the network is booted. When a new VM needs to be started thesystem does not have to add a new LID in the subnet, an action thatwould otherwise cause a complete reconfiguration of the network,including path recalculation, which is the most time consuming part.Instead, an available port for a VM is located (i.e., an availablevirtual function) in one of the hypervisors and the virtual machine isattached to the available virtual function.

In accordance with an embodiment, a vSwitch architecture withprepopulated LIDs also allows for the ability to calculate and usedifferent paths to reach different VMs hosted by the same hypervisor.Essentially, this allows for such subnets and networks to use a LID MaskControl (LMC) like feature to provide alternative paths towards onephysical machine, without being bound by the limitation of the LMC thatrequires the LIDs to be sequential. The freedom to use non-sequentialLIDs is particularly useful when a VM needs to be migrated and carry itsassociated LID to the destination.

In accordance with an embodiment, along with the benefits shown above ofa vSwitch architecture with prepopulated LIDs, certain considerationscan be taken into account. For example, because the LIDs areprepopulated in an SR-IOV vSwitch-enabled subnet when the network isbooted, the initial path computation (e.g., on boot-up) can take longerthan if the LIDs were not pre-populated.

InfiniBand SR-IOV Architecture Models—vSwitch with Dynamic LIDAssignment

In accordance with an embodiment, the present disclosure provides asystem and method for providing a vSwitch architecture with dynamic LIDassignment.

FIG. 8 shows an exemplary vSwitch architecture with dynamic LIDassignment, in accordance with an embodiment. As depicted in the figure,a number of switches 501-504 can provide communication within thenetwork switched environment 700 (e.g., an IB subnet) between members ofa fabric, such as an InfiniBand fabric. The fabric can include a numberof hardware devices, such as host channel adapters 510, 520, 530. Eachof the host channel adapters 510, 520, 530, can in turn interact with ahypervisor 511, 521, 531, respectively. Each hypervisor can, in turn, inconjunction with the host channel adapter it interacts with, setup andassign a number of virtual functions 514-516, 524-526, 534-536, to anumber of virtual machines. For example, virtual machine 1 550 can beassigned by the hypervisor 511 to virtual function 1 514. Hypervisor 511can additionally assign virtual machine 2 551 to virtual function 2 515,and virtual machine 3 552 to virtual function 3 516. Hypervisor 531 can,in turn, assign virtual machine 4 553 to virtual function 1 534. Thehypervisors can access the host channel adapters through a fullyfeatured physical function 513, 523, 533, on each of the host channeladapters.

In accordance with an embodiment, each of the switches 501-504 cancomprise a number of ports (not shown), which are used in setting alinear forwarding table in order to direct traffic within the networkswitched environment 700.

In accordance with an embodiment, the virtual switches 512, 522, and532, can be handled by their respective hypervisors 511, 521, 531. Insuch a vSwitch architecture each virtual function is a complete virtualHost Channel Adapter (vHCA), meaning that the VM assigned to a VF isassigned a complete set of IB addresses (e.g., GID, GUID, LID) and adedicated QP space in the hardware. For the rest of the network and theSM (not shown), the HCAs 510, 520, and 530 look like a switch, via thevirtual switches, with additional nodes connected to them.

In accordance with an embodiment, the present disclosure provides asystem and method for providing a vSwitch architecture with dynamic LIDassignment. Referring to FIG. 8, the LIDs are dynamically assigned tothe various physical functions 513, 523, 533, with physical function 513receiving LID 1, physical function 523 receiving LID 2, and physicalfunction 533 receiving LID 3. Those virtual functions that areassociated with an active virtual machine can also receive a dynamicallyassigned LID. For example, because virtual machine 1 550 is active andassociated with virtual function 1 514, virtual function 514 can beassigned LID 5. Likewise, virtual function 2 515, virtual function 3516, and virtual function 1 534 are each associated with an activevirtual function. Because of this, these virtual functions are assignedLIDs, with LID 7 being assigned to virtual function 2 515, LID 11 beingassigned to virtual function 3 516, and LID 9 being assigned to virtualfunction 1 534. Unlike vSwitch with prepopulated LIDs, those virtualfunctions 524-526 and 534-536 not currently associated with an activevirtual machine do not receive a LID assignment.

In accordance with an embodiment, with the dynamic LID assignment, theinitial path computation can be substantially reduced. When the networkis booting for the first time and no VMs are present then a relativelysmall number of LIDs can be used for the initial path calculation andLFT distribution.

In accordance with an embodiment, much like physical host channeladapters can have more than one port (two ports are common forredundancy), virtual HCAs can also be represented with two ports and beconnected via one, two or more virtual switches to the external IBsubnet.

In accordance with an embodiment, when a new VM is created in a systemutilizing vSwitch with dynamic LID assignment, a free VM slot is foundin order to decide on which hypervisor to boot the newly added VM, and aunique non-used unicast LID is found as well. However, there are noknown paths in the network and the LFTs of the switches for handling thenewly added LID. Computing a new set of paths in order to handle thenewly added VM is not desirable in a dynamic environment where severalVMs may be booted every minute. In large IB subnets, computing a new setof routes can take several minutes, and this procedure would have torepeat each time a new VM is booted.

Advantageously, in accordance with an embodiment, because all the VFs ina hypervisor share the same uplink with the PF, there is no need tocompute a new set of routes. It is only needed to iterate through theLFTs of all the physical switches in the network, copy the forwardingport from the LID entry that belongs to the PF of the hypervisor—wherethe VM is created—to the newly added LID, and send a single SMP toupdate the corresponding LFT block of the particular switch. Thus thesystem and method avoids the need to compute a new set of routes.

In accordance with an embodiment, the LIDs assigned in the vSwitch withdynamic LID assignment architecture do not have to be sequential. Whencomparing the LIDs assigned on VMs on each hypervisor in vSwitch withprepopulated LIDs versus vSwitch with dynamic LID assignment, it isnotable that the LIDs assigned in the dynamic LID assignmentarchitecture are non-sequential, while those prepopulated in aresequential in nature. In the vSwitch dynamic LID assignmentarchitecture, when a new VM is created, the next available LID is usedthroughout the lifetime of the VM. Conversely, in a vSwitch withprepopulated LIDs, each VM inherits the LID that is already assigned tothe corresponding VF, and in a network without live migrations, VMsconsecutively attached to a given VF get the same LID.

In accordance with an embodiment, the vSwitch with dynamic LIDassignment architecture can resolve the drawbacks of the vSwitch withprepopulated LIDs architecture model at a cost of some additionalnetwork and runtime SM overhead. Each time a VM is created, the LFTs ofthe physical switches in the subnet are updated with the newly added LIDassociated with the created VM. One subnet management packet (SMP) perswitch is needed to be sent for this operation. The LMC-likefunctionality is also not available, because each VM is using the samepath as its host hypervisor. However, there is no limitation on thetotal amount of VFs present in all hypervisors, and the number of VFsmay exceed that of the unicast LID limit. Of course, not all of the VFsare allowed to be attached on active VMs simultaneously if this is thecase, but having more spare hypervisors and VFs adds flexibility fordisaster recovery and optimization of fragmented networks when operatingclose to the unicast LID limit.

InfiniBand SR-IOV Architecture Models—vSwitch with Dynamic LIDAssignment and Prepopulated LIDs

FIG. 9 shows an exemplary vSwitch architecture with vSwitch with dynamicLID assignment and prepopulated LIDs, in accordance with an embodiment.As depicted in the figure, a number of switches 501-504 can providecommunication within the network switched environment 800 (e.g., an IBsubnet) between members of a fabric, such as an InfiniBand fabric. Thefabric can include a number of hardware devices, such as host channeladapters 510, 520, 530. Each of the host channel adapters 510, 520, 530,can in turn interact with a hypervisor 511, 521, and 531, respectively.Each hypervisor can, in turn, in conjunction with the host channeladapter it interacts with, setup and assign a number of virtualfunctions 514, 515, 516, 524, 525, 526, 534, 535, 536, to a number ofvirtual machines. For example, virtual machine 1 550 can be assigned bythe hypervisor 511 to virtual function 1 514. Hypervisor 511 canadditionally assign virtual machine 2 551 to virtual function 2 515.Hypervisor 521 can assign virtual machine 3 552 to virtual function 3526. Hypervisor 531 can, in turn, assign virtual machine 4 553 tovirtual function 2 535. The hypervisors can access the host channeladapters through a fully featured physical function 513, 523, 533, oneach of the host channel adapters.

In accordance with an embodiment, each of the switches 501-504 cancomprise a number of ports (not shown), which are used in setting alinear forwarding table in order to direct traffic within the networkswitched environment 800.

In accordance with an embodiment, the virtual switches 512, 522, and532, can be handled by their respective hypervisors 511, 521, 531. Insuch a vSwitch architecture each virtual function is a complete virtualHost Channel Adapter (vHCA), meaning that the VM assigned to a VF isassigned a complete set of IB addresses (e.g., GID, GUID, LID) and adedicated QP space in the hardware. For the rest of the network and theSM (not shown), the HCAs 510, 520, and 530 look like a switch, via thevirtual switches, with additional nodes connected to them.

In accordance with an embodiment, the present disclosure provides asystem and method for providing a hybrid vSwitch architecture withdynamic LID assignment and prepopulated LIDs. Referring to FIG. 9,hypervisor 511 can be arranged with vSwitch with prepopulated LIDsarchitecture, while hypervisor 521 can be arranged with vSwitch withprepopulated LIDs and dynamic LID assignment. Hypervisor 531 can bearranged with vSwitch with dynamic LID assignment. Thus, the physicalfunction 513 and virtual functions 514-516 have their LIDs prepopulated(i.e., even those virtual functions not attached to an active virtualmachine are assigned a LID). Physical function 523 and virtual function1 524 can have their LIDs prepopulated, while virtual function 2 and 3,525 and 526, have their LIDs dynamically assigned (i.e., virtualfunction 2 525 is available for dynamic LID assignment, and virtualfunction 3 526 has a LID of 11 dynamically assigned as virtual machine 3552 is attached). Finally, the functions (physical function and virtualfunctions) associated with hypervisor 3 531 can have their LIDsdynamically assigned. This results in virtual functions 1 and 3, 534 and536, are available for dynamic LID assignment, while virtual function 2535 has LID of 9 dynamically assigned as virtual machine 4 553 isattached there.

In accordance with an embodiment, such as that depicted in FIG. 8, whereboth vSwitch with prepopulated LIDs and vSwitch with dynamic LIDassignment are utilized (independently or in combination within anygiven hypervisor), the number of prepopulated LIDs per host channeladapter can be defined by a fabric administrator and can be in the rangeof 0<=prepopulated VFs<=Total VFs (per host channel adapter), and theVFs available for dynamic LID assignment can be found by subtracting thenumber of prepopulated VFs from the total number of VFs (per hostchannel adapter).

In accordance with an embodiment, much like physical host channeladapters can have more than one port (two ports are common forredundancy), virtual HCAs can also be represented with two ports and beconnected via one, two or more virtual switches to the external IBsubnet.

InfiniBand—Inter-Subnet Communication (Fabric Manager)

In accordance with an embodiment, in addition to providing an InfiniBandfabric within a single subnet, embodiments of the current disclosure canalso provide for an InfiniBand fabric that spans two or more subnets.

FIG. 10 shows an exemplary multi-subnet InfiniBand fabric, in accordancewith an embodiment. As depicted in the figure, within subnet A 1000, anumber of switches 1001-1004 can provide communication within subnet A1000 (e.g., an IB subnet) between members of a fabric, such as anInfiniBand fabric. The fabric can include a number of hardware devices,such as, for example, host channel adapter 1010. Host channel adapter1010 can in turn interact with a hypervisor 1011. The hypervisor can, inturn, in conjunction with the host channel adapter it interacts with,setup a number of virtual functions 1014. The hypervisor canadditionally assign virtual machines to each of the virtual functions,such as virtual machine 1 10105 being assigned to virtual function 11014. The hypervisor can access their associated host channel adaptersthrough a fully featured physical function, such as physical function1013, on each of the host channel adapters. Within subnet B 1040, anumber of switches 1021-1024 can provide communication within subnet B1040 (e.g., an IB subnet) between members of a fabric, such as anInfiniBand fabric. The fabric can include a number of hardware devices,such as, for example, host channel adapter 1030. Host channel adapter1030 can in turn interact with a hypervisor 1031. The hypervisor can, inturn, in conjunction with the host channel adapter it interacts with,setup a number of virtual functions 1034. The hypervisor canadditionally assign virtual machines to each of the virtual functions,such as virtual machine 2 1035 being assigned to virtual function 21034. The hypervisor can access their associated host channel adaptersthrough a fully featured physical function, such as physical function1033, on each of the host channel adapters. It is noted that althoughonly one host channel adapter is shown within each subnet (i.e., subnetA and subnet B), it is to be understood that a plurality of host channeladapters, and their corresponding components, can be included withineach subnet.

In accordance with an embodiment, each of the host channel adapters canadditionally be associated with a virtual switch, such as virtual switch1012 and virtual switch 1032, and each HCA can be set up with adifferent architecture model, as discussed above. Although both subnetswithin FIG. 10 are shown as using a vSwitch with prepopulated LIDarchitecture model, this is not meant to imply that all such subnetconfigurations must follow a similar architecture model.

In accordance with an embodiment, at least one switch within each subnetcan be associated with a router, such as switch 1002 within subnet A1000 being associated with router 1005, and switch 1021 within subnet B1040 being associated with router 1006.

In accordance with an embodiment, at least one device (e.g., a switch, anode . . . etc.) can be associated with a fabric manager (not shown).The fabric manager can be used, for example, to discover inter-subnetfabric topology, created a fabric profile (e.g., a virtual machinefabric profile), build a virtual machine related database objects thatforms the basis for building a virtual machine fabric profile. Inaddition, the fabric manager can define legal inter-subnet connectivityin terms of which subnets are allowed to communicate via which routerports using which partition numbers.

In accordance with an embodiment, when traffic at an originating source,such as virtual machine 1 within subnet A, is addressed to a destinationat a different subnet, such as virtual machine 2 within subnet B, thetraffic can be addressed to the router within subnet A, i.e., router1005, which can then pass the traffic to subnet B via its link withrouter 1006.

Overall, an example embodiment provides a very large fabric with manynodes and, further provides for multiple subnets each having a subnetboundary. The subnet boundaries enable independent subnet managers (SM).In particular, the subnet boundaries enable one independent SM in/foreach of the multiple subnets within the very large fabric.

In addition, very efficient packet routing is enabled by the use of thesimple lookup mechanism of the Linear Forwarding Table (LFT) lookupprotocol in order to route the packet flow in a unique way throughoutthe whole fabric.

Local/Intra-Subnet Routing:

For Layer 2 (L2) addressing, the example embodiment uses the “normal”local id based routing/forwarding within each subnet. That is, the16-bit Destination Local Identifier (DLID) in the Local Route Header(LRH) of the IB Packet is looked up when it is being routed at theswitch port and, the output port is quickly found by directly indexingthe LFT in hardware using the DLID value in the LRH of the IB packet.

Layer 3 (L3) Address/Addressing:

The Global Route Header (GRH) of packets in the example embodimentsessentially use the 128-bit scheme: the unique 64-bit node address—the“GUID,” and the 64-bit Subnet Prefix Number. The example embodimentprovides for a very large fabric that is divided into many independentsubnets, wherein the same route/routing capabilities are enabledspanning different subnets. That is, in the example embodiment, the pathbetween endpoints can be routed in the same way as if there was a largesingle subnet.

In virtual machines (VMs), the physical end nodes can represent a largenumber of virtual endpoints, wherein each of the virtual endpoints have(and should have for legacy compatibility reasons) their own complete IBaddress including the 16-bit DLID. However, the 16-bit DLID spacebecomes exhausted quickly. Essentially, there is more space in eachvirtual subnet than there is address space-pointing capability using the16-bit DLID.

In accordance with the example embodiment, therefore, the total fabricis divided into a set of independent subnets, a private “LID space” isprovided, and a mechanism is provided for extracting a designatedbit-field from the 64-bit Subnet Prefix Number portion of the L3address. In accordance with the mechanism of the example embodiment,instead of using the entire 64-bits of the L3 address to encode thesubnet number, the subject embodiment provides hardware support forextracting a designated bit-field—the ISRN—from the 64-bit Subnet PrefixNumber portion of the L3 address. Accordingly, when there is to becommunication from (virtual) Subnet A to (virtual) Subnet B through anintermediate infrastructure, the example embodiment encodes in thedesignated bit-field of the 64-bit Subnet Prefix Number portion of theL3 address both:

-   -   a) the LID in/of the destination subnet; and    -   b) an “intermediate” LID of an “intermediate switch fabric”        infrastructure.

Global Route Header (GRH) Based Linear Forwarding Table (LFT) Lookup

In order to reduce the amount of switch/router on-chip resourcesrequired to enable scalable, flexible and deterministicrouting/forwarding for both intra-subnet and inter-subnet traffic withina single fabric configuration with coordinated management ofrouting/forwarding throughout the whole IB fabric, a scheme is used inaccordance with an embodiment for identifying inter-subnet routes vianumbers that are used for linear table lookup in a manner the same asLID-based table lookup within a single subnet. In this regard, FIG. 11shows an illustration of a data packet format using the InfiniBand (IB)addressing scheme to access packet forwarding logic in a networkenvironment, in accordance with an embodiment. As shown in the figure, afirst IB subnet 1100 can include a plurality of physical (or virtual)nodes 1101 and 1102 and a subnet administrator (SA) 1120. A source node1101 within the first IB subnet 1100 can send a packet (e.g. an IBpacket 1110) to a destination node 1103 in a second IB subnet other thanthe first IB subnet 1100, via an intermediate node 1102. The destinationnode 1103 is shown in dashed lines to represent that it is in the secondIB subnet.

The IB packet 1110 can include the payload 1114 and various headersaccording to the IB protocols. These headers can include a globalrouting header (GRH) 1111, a local routing header (LRH) 1112, and otherheaders 1113. Additionally, the IB packet 1110 can be applied withvarious cyclic redundancy checks (CRCs) 1115.

In accordance with an embodiment, the system can take advantage of asubnet prefix portion of the destination global identifier (DGID) 1121in the GRH 1111 and the destination local identifier (DLID) 1122 in theLRH 1112 for supporting intra- and inter-subnet addressing in the firstIB subnet 1100 and the second IB subnet.

For example, the system can set the DLID 1122 in the IB packet 1110 tobe the DLID for the intermediate node 1102 (instead of the DLID for thedestination node 1103). Within the IB subnet 1100, the IB packet 1110can be routed via linear forwarding table LFT lookup to the intermediatenode 1102 based on the DLID 1122 as resolved by the SA 1120. Thus, theIB packet 1110 can be processed using packet forwarding logic providedon the intermediate node 1102. Similar packet forwarding logic can beprovided in the other nodes 1101 and 1103 as well as may be necessary ordesired.

Furthermore, the system can use selected portions of the DGID 1121 inthe GRH 1111 to indicate the DLID for the destination node 1103. Thus,the packet forwarding logic in the intermediate node 1102 is able toresolve (or obtain) the real DLID for the destination node 1103 based onthe DGID 1121 information in the GRH 1111.

In accordance with an embodiment, the intermediate node 1102 can performadditional packet header 1113 and/or payload 1114 modifications whennecessary. For example, the fabric level access control can be set up ina way that the source node 1101 and the destination node 1103 are eitherlimited members of a relevant partition or not members of the samepartition. In such a case, the intermediate node 1102 may be allowed tochange the P_Key value in the IB packet 1110 before forwarding themodified packet to the destination node 1103.

FIG. 11 shows the DGID portion 1121 of the exemplary packet 1100including subnet prefix forwarding portion 1131 including anInter-Subnet-Route-Number (ISRN) 1141 used in accordance with anembodiment together with a Target Subnet Number 1142 and a Target LID1143. The ISRN 1141 allows the same linear forwarding tables (LFTs) tobe used by fabric switches configured in accordance with an embodimentfor making both intra-subnet forwarding decisions as well as for makinginter-subnet forwarding decisions. The embodiment supports both fabriclocal/internal subnet numbers as well as “global” subnet numbers so thatinter-subnet traffic within a single fabric configuration can use adifferent subnet prefix value than traffic between different fabrics.

In order to enable the end-to-end route be completely defined in thepacket headers without any need to do any mapping from GRH.DGID to DLIDat any point along the path from source to final destination, anembodiment provides a GRH.DGID.subnetprefix format that includes boththe Inter Subnet Route Number, a target “fabric local subnet number” aswell as the destination LID value in the target subnet.

A scheme or protocol of forwarding domains associated with switch/routerports is supported in an example embodiment wherein each forwardingdomain type defines which address information is used for lookup in theLFTs, as well as if a change in forwarding domain type implies that thelocal route header should be updated with DLID values from the GRH.

The above DGID.subnetprefix format is used in accordance with anembodiment in order to support a three level forwarding domain structurewith a source subnet, an intermediate switch fabric and a target subnet.The source and intermediate nodes 1101, 1102 in the source subnet 1100use the initial LRH.DLID of the packet 1100 for linear forwarding table(LFT) lookup, whereas the transition to the destination node 1103 in theintermediate switch fabric implies that the Inter Subnet Route Number1141 of the subnet prefix forwarding portion 1131 used for LFT lookup.The intermediate switch fabric may be referred to herein also as “corefabric” for convenience.

In order to indicate that a packet is being forwarded within anintermediate switch fabric type forwarding domain, a special DLID valueis used in accordance with an embodiment, wherein the special DLID valuecan be matched against a value stored in a register in the ingressswitch port. The special DLID value represents a “Special Switch Port”wherein the special switch port defines an “end” of a local virtualsubnet. This allows the single LFT lookup operation to be based on therelevant section of the GRH.DGID.subnetprefix rather than the LRH.DLIDvalue.

In accordance with an embodiment and in order to be able to determinewhen a change of forwarding domain is to take place, each ingress switchport includes one or more registers with type status for each possibleegress port in the same switch node so that the packet processing in theingress path can prepare the packet for the next forwarding domain ifthe target port looked up in the LFT represents a change of forwardingdomain type.

In accordance with the example embodiment, in the case where the IBpacket 1110 is intra-subnet transferring from a port within the sourcesubnet 1100 such as from a port at the source node 1101 to a port alsowithin the source subnet 1100 such as to a port at the intermediate node1102, the LRH.DLID of the packet is directly used to index a LFT at eachswitch for standard intra-subnet packet transfer.

Further in accordance with the example embodiment, in the case where theIB packet 1110 is inter-subnet transferring from a port within thesource subnet 1100 such as from a port at the source node 1101 to theintermediate switch fabric outside of the source subnet 1100, the DLID1122 portion of the Local Route Header 1112 of the packet 1110 isupdated locally to a value that is configured to represent intermediateswitch fabric forwarding in the relevant intermediate switch fabricoutside of the source subnet 1100.

In accordance with yet a further example embodiment and with continuedreference to FIG. 11, in the case where the IB packet is inter-subnettransferring from the intermediate switch fabric outside of the sourcesubnet 1100 such as from the destination node 1103 to the target sourcesubnet 1100 such as to either the source node 1101 or the intermediatenode 1102, the LRH.DLID is updated with the DLID field from theDGID.subnetprefix and is then used to forward the packet to the finaldestination port using LFT lookup in the target source subnet 1100. Inthis transition, it is also possible to verify that the target localsubnet number in the packet GRH.DGID.subnetprefix is matched with localsubnet number that this port represents. This checking is preferablybased on a register in each such port.

In an example embodiment, any switch port can represent a “gateway”between the local (source) subnet and the intermediate switch fabric andeach such gateway switch port is identified with one or more specificDLIDs in the local subnet. It is therefore possible to implementfat-tree based topologies where multiple parallel core-fabrics canexist. In this way the addressing scalability of the complete fabric canscale with the topology since each parallel core fabric can represent anindependently routed domain. The 48K LFT size is then only a limitationwithin each of these parallel core fabrics, and the number of individual“inter subnet route numbers” within the complete core fabric is 48Ktimes the number of parallel core fabrics.

In addition and in accordance with an embodiment, in order to be able tohandle “random” connectivity between different fabric configurationswith independent management of internal routes, an embodiment provides asupport scheme for supporting both fabric local/internal subnet numbersas well as “global” subnet numbers so that inter-subnet traffic within asingle fabric configuration can use a different subnet prefix value thantraffic between different fabrics. The support scheme also provides TCAMbased lookup and mapping of complete DGID values in order to allowforwarding of arbitrary subnet prefix and GUID values.

Fabric Model

As an overview of the core fabric model in accordance with the exampleembodiment, the IB fabric configuration facilitates scaling to verylarge fat-tree configurations that can be decomposed into individualsubnets, while still maintaining the flexibility of being able toexplicitly route individual paths for individual physical or virtualdestinations. The system is constructed as a single fabric configurationthat is managed in a hierarchical manner, but where a top levelmanagement entity has full visibility of the complete fabric.

In order to provide large system construction flexibility while stillsupporting native IB connectivity between different independentlymanaged systems, the fabric model of the example embodiment enables“arbitrary” IB connectivity between different fabricconfigurations/topologies that are not managed by the same single toplevel entity.

The example embodiment encodes path/route information as part of thesubnet prefix portion 1131 of the DGID field in the packet GRH forachieving the above goals and also for providing inter-subnetcommunication within a single fabric configuration. The exampleembodiment also supports a standard subnet prefix scheme that enableIB-IB routing between independent IB fabrics.

In addition to scaling to very large configurations with hundreds orthousands of physical hosts, the system of an example embodiment alsofacilitates use of very small subnet configurations where a singleintegrated circuit chip may be used to implement multiple virtualswitches that each represent a different IB subnet managed by adifferent set of Subnet Managers (SMs). In addition, even for very largetotal fabric configurations, such small subnet configurations may stillbe used as necessary or desired in order to reflect the desireddecomposition into logical system units. On the other hand, someconfigurations may have very large single subnet topologies that arestill part of a larger single fabric.

In general, the need for flexibility in up-scaling as well asdown-scaling imposes a challenge in terms of system on-chip resourcesfor handling packet routing and forwarding decisions. This includes bothintra-subnet and inter-subnet forwarding, and in particular the abilityto address individual virtual end-points in different subnets.

Embodiments herein provide schemes for reducing the amount of systemon-chip resources required to enable scalable, flexible anddeterministic routing/forwarding for both intra-subnet and inter-subnettraffic within a single fabric configuration. In particular, an“Inter-Subnet-Route-Number” (ISRN) can be used for identifyinginter-subnet routes via numbers that can be used for table lookup in amanner similar to LID based table lookup within a single subnet. Inaddition, otherwise standard or “legacy” linear forwarding tables areused for both intra- and inter-subnet forwarding decisions. For example,FIG. 12 shows a portion of a Linear Forwarding Table (LFT) 1200 that maybe indexed from 0-48k equivalently by either the LRH.DLID value or bythe ISRN value. In the LFT 1200 illustrated, each entry in the tablecontains either a supported port number or a “0xFF” entry representingan entry not used. In further addition, both fabric local/internalsubnet numbers as well as “global” subnet numbers are supported so thatinter-subnet traffic within a single fabric configuration can use adifferent subnet prefix value than traffic between different fabrics.Yet still further, a GRH.DGID.subnetprefix format is provided andsupported. In the example embodiment, the GRH.DGID.subnetprefix formatincludes both an “inter subnet route number”, a target “fabric localsubnet number” as well as the destination LID value in the target subnetin order to enable the end-to-end route be completely defined in thepacket headers without any need to do any mapping from GRH.DGID to DLIDat any point along the path from source to final destination.

Leaf-switch based forwarding is provided in order to allow ahierarchical topology with “randomized” multi-path forwarding betweenindividual subnets. In addition, in order to able to handle “random”connectivity between different fabric configurations, the exampleembodiment supports both fabric local/internal subnet numbers as well as“global” subnet numbers so that inter-subnet traffic within a singlefabric configuration can use a different subnet prefix value thantraffic between different fabrics, and provides TCAM based lookup andmapping of complete DGID values in order to allow forwarding ofarbitrary subnet prefix and GUID values.

Local Fabric

In accordance with an embodiment, a “Local Fabric” is a physical IBfabric infrastructure that represents connectivity within and betweenone or more physical “leaf subnet” and/or “core fabric” topologies.Local fabrics are managed in a hierarchical manner, but with theconstraint that a top-level central management entity such as forexample control-plane software, is capable of managing any ID valueassignment and observe both physical connectivity and routing (packetforwarding) constraints throughout the complete local fabric (i.e. a“holistic view” of the local fabric).

Remote Fabric

In accordance with an embodiment, a “Remote Fabric” is any IB fabrictopology/infrastructure that is connected to a local fabric via one ormore IB links but where the port on the remote side of the link is notcontrolled by the same top level central management entity as the localfabric. The ports connecting two independent local fabrics are typicallyconfigured as router ports. Hence, these two ports form, in accordancewith an embodiment, a minimal IB subnet.

Gateway Subnet

In accordance with an embodiment, a “Gateway Subnet” is a “trivial”physical IB subnet that is typically formed by connecting one routerport from one local fabric with exactly one router port from anotherlocal fabric (i.e. no intermediate switch). The gateway subnet has atrivial configuration that is established at link-up time and packetforwarding policy is controlled independently by the routers on eachside. The two router ports implement a mutual discovery protocol inorder to learn about the remote fabric. Alternatively and/oradditionally, administrative input in each local fabric can selectivelybe used as necessary or desired to define a set of one or more remotesubnet numbers and individual GIDs that should be routed throughdifferent Gateway Subnets. A significant aspect of the gateway subnetdefinition herein is that it places the boundary between the two toplevel administrative domains on an IB link rather than on a router orswitch device that has to be managed.

More complex Gateway subnets imply that switches within the gatewaysubnet also require management and this management introduces complexityand policy issues that are not present or are significantly less presentin the direct link case. Hence, rather than having a well-defined ownerof each side of a single link, the question of ownership of the gatewaysubnet becomes an issue. As with any large scale networking, it ishowever possible to imagine more complex configurations where multiplelocal fabrics are connected via one or more other local fabrics (i.e.the admin of the intermediate local fabric determines how the differentconnected remote fabrics are allowed to communicate via this localfabric. The embodiments herein are applicable to this more complexgateway model since each local fabric may have connectivity to multipleother local fabrics. In any case, even with these types of intermediatelocal fabrics, the boundary between any pair of local fabrics ispreferably a minimal gateway subnet as defined above.

Leaf Subnet

A “Leaf Subnet” is a physical IB subnet that has its own unicast andmulticast LID spaces. Host type end-nodes such as compute and storageservers are typically connected to the local fabric via a single leafsubnet. However, a single host may also be connected with multiple leafsubnets as desired. A leaf subnet is configured and controlled by one ormore cooperating Subnet Manager (SM) instances and, within a leafsubnet, packets are forwarded based on LRH.DLID portions of the packetheader. A leaf subnet may consist of legacy switches, and is identifiedby an administratively defined “Identification Subnet Number” (ISN) thatrepresents policy input to the set of Subnet Managers that arecontrolling the leaf subnet, as well as the central top-level managemententity. The ISN may be globally unique, and is preferably unique acrossall local fabrics that can be directly connected via IB links, i.e. viathe Gateway Subnets. Each leaf subnet may be connected to one or more“core fabrics” via ports, and each leaf subnet is associated with atleast one “Fabric Local Subnet Number” (FLSN) when it is connected to atleast one core fabric. The FLSN value has scope for the local fabric andis allocated by the local fabric management infrastructure. In addition,a leaf subnet may be connected to other IB subnets via router ports thatdo not represent core fabric connectivity. In the general case, such“global” connectivity represents connectivity to a remote fabric via agateway subnet and, for this kind of global connectivity, the ISN valueis used for DGID based addressing of this leaf subnet from remotesubnets/fabrics. However, it is also possible to connect two leafsubnets in the same local fabric in this way i.e. without a local corefabric. This kind of connectivity may use either ISN or FLSN based DGIDaddressing. Router port based connectivity directly from a leaf subnet,either within the local fabric or to a remote fabric, may involve eithersystem ports or other router implementations.

Core Fabric

In accordance with an embodiment, a “Core Fabric” is topology inaccordance with the example embodiment that connects one or more leafsubnets within the context of a single local fabric. The expression“intermediate switch fabric” may be used alternatively herein. Inaccordance with an embodiment, a core fabric provides additionalconnectivity between different sections of a single leaf subnet such asfor example sections that already have direct connectivity within thesame physical subnet. In the example embodiment, preferably, a singlecore fabric represents a single “Inter Subnet Route Number” (ISRN) valuespace. A single core fabric can have directly connected end-nodes. Aspecial case of end-nodes is switch management ports (port 0). These canbe reached via a dedicated ISRN value and requires GRH basedcommunication. Physical end-nodes connected via physical IB links and/orsystem ports in general represent a separate FLSN (or ISN) value andcommunication is preferably GRH-based. Such directly connected end-nodesthen in general represent a “trivial” leaf subnet with only two portsand a single link, i.e. the end-node port and the relevant system port.It is to be appreciated that, in the environment of the exampleembodiment, a directly connected physical end-node can also be addressedvia a dedicated ISRN value as long as the end-node implementation isable to handle that the DLID value may not be unique. In order to reducerequired value space since FLSN values in general have to be global forthe whole local fabric, separate FLSN values might selectively not beallocated for such end-ports.

A single core fabric, in accordance with an example embodiment,processes or otherwise handles Direct Route (DR) Subnet ManagementPackets (SMPs) in a manner similar to other processing or handlingwithin a conventional physical IB subnet. In accordance with anembodiment, however, a single core fabric is configured to support LIDbased forwarding as well as ISRN based forwarding. Accordingly, thetotal LID value space used is coordinated with the total ISRN valuespace used. The coordination includes using different sections of thesame physical LFT resources for both types of forwarding, andconfiguring the size of these sections as a run-time configurationparameter for each system port.

In an example embodiment, there are no restrictions on overlapping LIDand ISRN values as long as the sum of the two used value spaces can besupported by the involved hardware components. Assuming a sufficient LIDvalue space is allocated, both system Port 0 instances as well asdirectly connected end-nodes can be reached via LID based addressing. Inthis way, a single core fabric instance can also represent aconventional IB subnet and represent a destination leaf subnet within alocal fabric at the same time as it is functioning as a core fabricbetween one or more (other) leaf subnets.

In addition, in an example embodiment, a single core fabric can havedirect physical connectivity to one or more other single core fabricswhile still maintaining independent ISRN value spaces for each suchsingle core fabric. DR SMPs may be used to communicate between the twocore fabric instances without depending on any other type of forwardingbeing supported. Based on coordination of value spaces and reservationof a specific subset of either LID or ISRN space, communication betweenall or a subset of the end nodes in two core fabrics can be allowed oris otherwise enabled using either LID or ISRN based addressing.

A single core fabric may have, in an example embodiment, one or morerouter ports that represent connectivity to remote fabrics. That is,they are independent local fabrics without any coordinated FLSN or ISRNspaces. Such remote connectivity is preferably implemented using GatewaySubnets. In this case, forwarding of packets to/from local end nodes orconnected leaf subnets is based on GRHs with ISN based subnet prefixvalues as well as the associated GUID value in the DGID field. Suchforwarding does not depend on any specific formats for any part of theDGID, and instead relies on TCAM based forwarding.

Addressing Mode

In accordance with an embodiment, the “Addressing Mode” of a packetdefines the particular header information to be used to determine howthe packet should be forwarded from source to destination. An IB packetmay include multiple headers (i.e. LRH and GRH) that contain informationabout how the packet should be forwarded from source to destination.Thus, the addressing mode of a packet defines what such information tobe used for the next forwarding decision.

In an example embodiment, the relevant addressing modes in this contextincludes the following:

-   -   LRH/DLID Addressing Mode;    -   GRH/ISRN Addressing Mode;    -   GRH/LeafSwitch Addressing Mode; and    -   GRH/DGID Addressing Mode.

Forwarding Mode

In accordance with an embodiment, “Forwarding Mode” applies to switchports that perform the packet forwarding decisions. While the“addressing mode” concept applies to the headers of an IB packet, theswitch ports that perform forwarding decisions operate in a “forwardingmode” that corresponds to some packet addressing mode. A switch port maysupport more than one addressing mode concurrently. Hence, in order toallow a switch port to select the correct forwarding mode for aparticular packet, the packet may have to contain information thatallows the switch port to deterministically determine the addressingmode. Such information includes both DGID type information as well asspecial DLID values that have been reserved to denote a specificforwarding mode (e.g. GRH/ISRN). Also, in some cases, the next hopaddressing mode is a function of both address information in the packetas well as configuration information associated with ports in thefabric. The ability in the example embodiment to determine when thedestination leaf subnet has been reached is an example of the next hopaddressing mode being a function of both address information in thepacket as well as configuration information associated with ports in thefabric.

Forwarding Domains

In accordance with an embodiment, any specific Leaf Subnet, Core Fabricor Gateway Subnet instance may represent a “Forwarding Domain.” As an IBpacket is forwarded through the complete IB fabric, it passes throughone or more “forwarding domains” such as shown in FIG. 13 where eachsuch forwarding domain represents a specific forwarding mode as well asa specific address value space. In accordance with an embodiment,“Forwarding Domains” are boundaries between multiple Leaf-Subnets andmultiple Core Fabrics. In the most straight forward scheme, anyindividual system instance will belong to a single forwarding domain,e.g. a single leaf-subnet or single core-fabric instance. In this case,all boundaries between any pair of leaf-subnet or core-fabric instancesexist on the one or more links that connect different system instances.In order to allow both leaf-subnets and core-fabric instances to havearbitrary sizes, individual system instances are selectively dividedinto two or more “virtual switch” instances where each such virtualswitch represents a collection of one or more “virtual switch ports”that all represent connectivity to/within either a single leaf-subnet ora single core-fabric. In this case, the boundary between any pair offorwarding domains will exist within the system crossbar that connectstwo or more virtual switch instances.

As shown in FIG. 13, within a Source Subnet, LFT lookup is performed ina first forwarding domain using the original (or legacy) LRH.DLIDscheme. In a second forwarding domain and as shown, LFT lookup withinthe Core Fabric and when forwarding packets from a Source Subnet to theCore Fabric, is performed using the ISRN scheme in accordance with theexample embodiment herein. Further and as shown, LFT lookup in a thirdforwarding domain when forwarding packets from the Core Fabric to theTarget Subnet is performed using the Target LID 1143 (FIG. 11) basedLRH.DLID.

Gateway Port

In accordance with an embodiment, a physical system port that representsconnectivity to a forwarding domain is referred to herein as a “gatewayport.” For the reasons set out below, a gateway port is defined from theperspective of another port on the same system instance sending packetsto the gateway port. First, sending packets to a gateway port impliesthat the addressing mode for the packet is changed because the sendingis one of:

-   -   I. sending source-leaf to core-fabric,    -   II. sending core-fabric to destination-leaf, or    -   III. sending source-leaf to destination-leaf.

Since the addressing mode of a packet applies to how forwardingdecisions are made for packets received from the IB link, the change ofaddressing mode when crossing from one forwarding domain shown in FIG.13 to another is, in accordance with an embodiment, implemented as thepacket is moved from the system ingress port to the egress port such asmay be, for example, the gateway port. The addressing mode will then beactive when the packet is forwarded to the IB link from thegateway/egress port.

For this reason, the gateway port is defined from the perspective ofanother port on the same system instance sending packets to the gatewayport.

It is to be appreciated that a link that represents a boundary betweendifferent forwarding domains will normally have gateway ports on bothsides of the link. The motivation for having gateway ports on both sidesof the link is to ensure that forwarding policy updates are contained oneach side of the link, and thereby remove the need for any write typemanagement operations across the link. For example, if one side of thelink represents a core-fabric and the other side of the link representsa leaf-subnet, then the subnet manager on the leaf-subnet side has inaccordance with the example embodiment complete control of forwarding ofthe packets received on the IB link.

In the example embodiment, packets on the link typically have differentaddressing mode in each direction, and the forwarding mode for the portson each side of the IB link will correspond to the “forwarding domain”on that side of the link. For example, if one side of the linkrepresents a core-fabric and the other side of the link represents aleaf-subnet, then packets being forwarded from the core-fabric to theleaf-subnet will have LRH/DLID-based addressing mode as shown on theright in FIG. 13, and the packets being forwarded from the leaf-subnetto the core fabric will have GRH/ISRN-based addressing mode as shown onthe left in FIG. 13.

It is to be noted that in the case of a single gateway port located ononly one side of the link, that single gateway port is to be managed aspart of the forwarding domain on the remote side of the link. As anexample, if one side of the link represents a core-fabric and the otherside represents a leaf-subnet, and if there is no gateway port on theleaf-subnet side, then the port on the core-fabric side of the linkwould also need to be part of the leaf-subnet and, further, to have itsforwarding logic configured by the subnet manager in the leaf-subnet.Since this implies that this subnet manager then effectively implementsthe first hop forwarding within the core fabric, an unnecessarydependency between the two forwarding domains can be effectivelyeliminated by moving the boundary between the forwarding domains fromthe link to the system crossbar where the gateway port is located on thecore-fabric side. The above is followed correspondingly relative toforwarding for the packets in the other direction.

On the other hand, with gateway ports on both sides of the link, theonly dependency between the forwarding domains is that the forwarding ofpackets through the gateway port and onto the IB link is performed withan assurance that the packet has or is provided with the correctaddressing mode. Since the output port for the packet is already decidedwhen the change of addressing mode is performed, the change ofaddressing mode has no impact on forwarding/routing decision at thispoint.

On the receiving side, the ingress packet handling and forwardingdecisions is performed in accordance with a standard intra-subnetprotocol just as for any switch port in that forwarding domain.

In the case of crossbar based boundaries within a single systeminstance, any physical port that is a gateway port relative to anyvirtual switch instance on the same crossbar, i.e. within the samesystem instance) may also be a virtual switch port within anothervirtual switch within the same system instance.

A special case involves a single port connecting a system instance tosome leaf-subnet or core-fabric. Although technically a single port isby definition not a switch in the sense that it can perform packetforwarding between other source and target end-ports within the localsubnet/forwarding-domain, from an IBTA perspective a switch with only asingle external port is still a valid construct in that it providesaccess to the switch management port and thereby allows the switchmanagement port to function as an end-point within the subnet. In thesubject example embodiment, such a single port is still relevant forswitching since it provides access via the crossbar based forwardingdomain boundaries to other leaf-subnets and/or core-fabrics. Hence, fromthe perspective of the remote leaf-subnet or core-fabric it providesaccess to a number of gateway ports, but not to any virtual switch portwithin the local forwarding domain.

In addition to the above, it is to be noted that if more than onephysical port connects a system instance to the same core-fabric orleaf-subnet, then this collection of ports would by default berepresented as a virtual switch instance. However, from a fabrictopology perspective, there may be no need for this switchedconnectivity, and it will be a matter of fabric definition policy torepresent the collection of ports as one or more virtual switches,including the case of all individual ports only providing connectivity,to gateway ports in other leaf-subnets or core fabrics without allowingconnectivity to other ports in the same forwarding domain.

Leaf-Subnet Virtual-Switch

In accordance with an embodiment, a “Leaf-Subnet Virtual-Switch”defines, as a baseline, a collection of one or more physical ports on asingle system instance that connects to the same leaf subnet. If FabricLocal Subnet Numbers (FLSNs) are in use, then all the ports in the sameleaf-subnet virtual-switch will normally represent the same FLSN value.It is to be noted that, conceptually, different ports within in the sameleaf-subnet virtual-switch may represent different FLSN values and eachport may represent more than one value even though they all representthe same physical subnet. Also, the same FLSN value may be associatedwith leaf-subnet virtual-switch ports in different physicalleaf-subnets.

One motivation for the above schemes is to allow VM migration betweenleaf subnets without implying a change of path info. However, in orderto maintain the complete path information, the DLID can be preserved ora mapping function can be provided for the DLID that can be used in suchexception cases.

With a restricted number of mapping resources, as well as a restrictednumber of more than one FLSN values per port, an advanced scheme inaccordance with a further example embodiment is to use a two-stepmigration scheme where the migration is performed without any pathupdate in the first round using some of the restricted hardwareresources, while providing for a concurrent path update process so thatthe restricted hardware resources could be released again once the pathinformation has been updated by all relevant peers and the originalpath/address is no longer needed.

In an example embodiment, a leaf-subnet virtual-switch presents itselfas a standard IB switch via standard SMA attributes. Hence, legacySubnet Managers (SMs) are able to discover and configure a system basedleaf-subnet virtual-switch in a manner similar to any other standard IBlegacy switch that is part of the local subnet.

Further in an example embodiment, a leaf-subnet virtual-switch may beextended to represent connectivity to other parts of the local fabric orto a gateway subnet providing connectivity to a remote fabric. Suchconnectivity implies that one or more “special” ports are included inthe leaf-subnet virtual-switch and can be observed directly such as, forexample, as additional port numbers on the same virtual switch viaspecial SMA attributes. That is, one or more core-fabric gateway portscan provide connectivity to one or more core fabrics, and one or morevirtual router ports can provide connectivity to physical router portsas well as to various other ports outside the scope of the leaf-subnetvirtual-switch.

For example, A virtual router port can be observed indirectly viastandard SMA attributes, but then be represented by another switch portthat has a (virtual) link to another port that is identified as a routerport.

In this way, a legacy Subnet Manager (SM) is able to discover andutilize router based connectivity to other subnets. It is to beappreciated, however, that a system aware Subnet Manager (SM) will makeuse of the special SMA attributes to discover special ports directly, sothe need for “legacy SM” support is diminished.

Further with regard to the example embodiment, one or more leaf-subnetgateway ports can provide connectivity to one or more other leafsubnets.

Leaf Subnet Virtual-Switch Port

In accordance with an example embodiment, a leaf-subnet virtual-switchport can belong to a leaf-subnet virtual-switch instance, and uses theLRH.DLID value in order to look up output port number such as forexample for packets arriving on the IB link. The type of port the outputport number represents relative to this leaf-subnet virtual-switchinstance is defined in the example embodiment by configurationinformation available to the member physical ports. The output port mayrepresent another virtual switch port in the same leaf-subnetvirtual-switch, a core fabric gateway port, a virtual router port or aleaf-subnet gateway port providing direct connectivity to another leafsubnet. The output port may in general not be a physical router portbecause this implies that a new LRH will be constructed in a dynamicfashion based on the packet GRH, and this can only be done via a virtualrouter port.

It is to be appreciated that, given the definition of simplified GatewaySubnets with only two ports (i.e. local and remote router port), it ispossible to provide a simplified scheme where the DLID to use for thenext hop is based on either a fixed value, or would be based on havingthe local physical router port record the LID value of the remote routerport, and ensure that this is used as the next hop DLID.

It is to further be appreciated that a physical port that is aleaf-subnet virtual-switch port may also operate as a Leaf SubnetGateway Port from the perspective of another leaf-subnet virtual-switchor core-fabric virtual-switch.

Core-Fabric Virtual-Switch

In accordance with an embodiment, a “Core-Fabric Virtual-Switch”defines, as a baseline, a collection of one or more physical ports on asingle system instance that connects to the same core fabric.

A core-fabric virtual-switch presents itself in the example embodimentas a special IB switch via selected special SMA attributes. Hence, alegacy Subnet Manager (SM) will not be able to discover and configure acore-fabric virtual switch of the system of the example embodiment.

A core-fabric virtual-switch may be extended to represent connectivityto other parts of the local fabric or to a gateway subnet providingconnectivity to a remote fabric. Such connectivity implies that one ormore other “special” ports are included in the core-fabricvirtual-switch and can be observed directly, for example as additionalport numbers on the same virtual switch via special selected SMAattributes, wherein one or more leaf-subnet gateway ports can provideconnectivity to one or more leaf subnets, one or more virtual routerports can provide connectivity to physical router ports as well as tovarious other ports outside the scope of the leaf-subnet virtual-switch,and one or more core-fabric gateway ports can provide connectivity toone or more other core fabrics.

Core-Fabric Virtual-Switch Port

In accordance with the example embodiment, a core-fabric virtual-switchport of a core-fabric virtual-switch instance uses the GRH.DGID.ISRNvalue in order to look up output port for packets arriving on the IBlink. The type of output port the looked up port number representsrelative to this core-fabric virtual-switch instance is defined byconfiguration information available to the member physical ports. Theoutput port may represent another core-fabric virtual-switch port in thesame core-fabric virtual-switch, a leaf-subnet gateway port, a virtualrouter port, or a core fabric gateway port providing direct connectivityto another core fabric.

The output port may in general not be a physical router port becausethis implies that a new LRH has to be constructed in a dynamic fashionbased on the packet GRH, and this can only be done via a virtual routerport.

It is to be appreciated that a physical port that is a core-fabricvirtual-switch port may also operate as a Core Fabric Gateway Port fromthe perspective of another leaf-subnet virtual-switch or core-fabricvirtual-switch.

Core-Fabric Gateway Port

It is to be appreciated that in the case of system crossbar basedvirtual switch and forwarding domain boundaries, a physical port that isa core-fabric gateway port may also operate as a core-fabricvirtual-switch port from the perspective of the core-fabricvirtual-switch that represents the relevant core fabric connectivity.Hence, from the perspective of the physical switch port, the packetforwarding alternatives include those described for “Core-FabricVirtual-Switch Port” above in addition to the gateway port relatedforwarding described in this section.

In accordance with an example embodiment, a core-fabric gateway portrepresents a port that handles ingress to or egress from a core fabricinstance relative to one or more leaf-subnet virtual-switch port(s) orcore-fabric virtual-switch port(s) belonging to other core-fabricvirtual-switches in the same system instance such as for example wherethose virtual core fabric switches also represent other core fabricinstances/forwarding domains.

A core fabric gateway port receiving packets via the system crossbarcan, in general, not change any addressing mode, and instead rely onthat the received packet already has been set up with correct addressingfrom the system ingress port that forwarded it.

It is to be noted that in an example embodiment, since the physical portmay have multiple roles and the specific role is decided at the point offorwarding the packet from the ingress port, there is in general no needfor updating address information at the egress point unless somehardware design constraints makes this kind of pipelining moreefficient.

In the case of link-based forwarding domain boundaries, a core fabricgateway port is part of the forwarding domain of the leaf-subnet thelocal system instance represents. Hence, the forwarding mode for packetsarriving on the IB link is the “LRH/DLID” forwarding mode.

In the case of system crossbar based forwarding domain boundaries, acore fabric gateway port is part of the forwarding domain defined by theremote core fabric instance. Hence, the forwarding mode for packetsarriving on the IB link is the “GRH/ISRN” forwarding mode.

The type of output port the looked up port number represents relative tothe relevant forwarding domain (i.e. the core-fabric instance thegateway port represents) is defined by configuration informationavailable to the physical port (i.e. same information as outlined forthe virtual-switch port personality, if any, of the physical port).

The output port may represent a leaf-subnet gateway port, or anothercore-fabric gateway port such as might be the case when two core fabricsare connected via a single system crossbar, and may also represent avirtual router port.

FIG. 14 shows an illustration of modifying a packet header from anGRH/ISRN addressing mode format to a LRH/DLID based forwarding format,in accordance with an embodiment. When packets move from the core-fabricgateway port via the system crossbar to a leaf-subnet gateway port, thenthis transition represents changing from GRH/ISRN addressing mode toLRH/DLID based forwarding. This implies that, as shown in the figure, anew LRH is constructed in the header of a new packet 1400 with the DLID1143 extracted from the packet GRH 1111 (i.e., the system specificGRH.DGID.remoteDLID field). It is to be noted that, in general, thisforwarding is well defined independently of correlating the packet FLSNvalue with the FLSN value associated with the leaf subnet gateway port.That is, the only information needed in order to accomplish the changein addressing mode is that the egress port is defined as a leaf-subnetgateway port from the perspective of the ingress port, and this isindependent of what FLSN value is involved. Based on this observation,any correlation of packet FLSN value with the port FLSN value isessentially an “assert” of that the routing is set up correctly.

This issue is similar to that of the LID(s) associated with end nodes.The local LID(s) are needed in order to generate a correct SLID whensending packets, but as long as the subnet is routed correctly accordingto LID assignment, there is essentially no need to handle the DLID inincoming packets once the packet has arrived at the destinationend-node. As long as switch forwarding tables are updated in correctsequence relative to assigning initial or new LID values to end nodes,the end nodes will never observe a case where the received DLID is inconflict with the assigned LID.

On the other hand, making DLID check optional in the end-node impliesthat the fabric in principle can use any number of LIDs and associatedroutings for forwarding to a single end node. In this case where thedestination is a leaf subnet, making the FLSN value check optionalimplies that VM migration between different leaf subnets can beimplemented with more flexibility.

FIG. 15 is an illustration of a switch providing a mechanism defining aspecial switch port boundary for changing packet forwarding from anLRH/DLID addressing mode to a GRH/ISRN addressing mode, in accordancewith an embodiment. When packets move from a leaf-subnet gateway portvia the system crossbar to the core-fabric gateway port, then thistransition represents changing from the LRH/DLID addressing mode to theGRH/ISRN addressing mode. This implies that, as shown in the figure, anew LRH is constructed with a fixed DLID identifying ISRN basedforwarding. In this regard, a switch 1500 provides a mechanism defininga special switch port boundary for changing packet forwarding from anLRH/DLID addressing mode to a GRH/ISRN addressing mode. In accordancewith an embodiment supporting ingress into the intermediate switchfabric, a mechanism is provided in selected switches 1500 of the fabricwhereby a boundary can be defined. In an embodiment, a “Special SwitchPort” (SSP) 1510 is defined, wherein the SSP represents an “end” of alocal subnet. In an embodiment, the packet uses the DLID 1122 (FIG. 11)in the original/normal Local Route Header 1112 to point to the SSP 1510via the LFT 1200 (FIG. 12) representative of a virtual end port of thevirtual subnet. The SSP is provided with an attribute indicating that atthis point it represent an “End Port” in this subnet. The SpecialAttribute indicates a change in mode of forwarding—wherein at the nextlevel of forwarding, instead of using the DLID 1122 in the Local RouteHeader 1112, it uses the designated/dedicated bit-field 1131 from the64-bit Subnet Prefix Number portion of the GRH/L3 address. Essentially,the original DLID 1122 is replaced with the designated/dedicated bitfield, or a portion thereof, to form a new DLID 1110′ for use in ISRNbased forwarding in accordance with the example embodiment.

In an embodiment, the switch 1500 can use a LID-based filteringtechnique. In this embodiment, the switch 1500 can include a receiving(Rx) filter 1520, which can identify packets targeting the SSP 1510.Thus, the switch 1500 can separate the data flow traffic targeting theSSP 1510 from the data flow traffic targeting the normal/legacyforwarding modality ports 1530. For example, the Rx filter 1520 canseparate the mixed data flow traffic based on the service DLID (e.g.using DLID based filtering). The following is an exemplary packetforwarding logic DLID table:

-   -   DLID=0xF    -   DLID=0xFF

When an incoming packet has a matching DLID, (e.g. 0xF or 0xFF), the Rxfilter 1520 can direct the packet to the SSP 1510. On the other hand, ifan incoming packet does not have a matching DLID (i.e. with a DLID otherthan 0xF and 0xFF), the Rx filter 1520 can direct the incoming packet tothe normal/legacy forwarding modality ports 1530, which can use an IBprotocol engine to handle the IB packet according to the standard IBprotocol.

In accordance with an example embodiment, when packets move between thecore-fabric gateway port and another core-fabric gateway port (i.e.connected to a different core fabric instance) then the sameGRH.DGID.ISRN value is still used for forwarding. Hence, the managementsoftware ensures that packet traffic crossing from one core fabricinstance to another is representing a GRH.DGID.ISRN value that ishandled consistently in both the two or more core fabrics since bydefault, each core fabric instance represents its own ISRN value space.

When packets move from the core-fabric gateway port to a virtual routerport, then the next step forwarding scheme is a function of the policiesimplemented by the virtual router port.

The output port may in general not be a physical router port becausethis implies that a new LRH is to be constructed in a dynamic fashionbased on the packet GRH, and this is performed via a virtual routerport.

Leaf-Subnet Gateway Port

In the case of a system crossbar based forwarding domain boundaries, aphysical port that is a leaf-subnet gateway port may also operate as aleaf-subnet virtual-switch port from the perspective of the leaf-subnetvirtual-switch that represents the relevant leaf-subnet connectivity.Hence, from the perspective of the physical switch port, the packetforwarding alternatives includes those described for “Leaf-SubnetVirtual-Switch Port” above in addition to the gateway port relatedforwarding described in this section.

In accordance with an example embodiment, a leaf-subnet gateway portrepresents a port that handles ingress to or egress from a leaf-subnetinstance relative to one or more core-fabric gateway ports and/or one ormore leaf-subnet virtual-switch ports belonging to other leaf-subnetvirtual-switches in the same system instance. In this regard, aleaf-subnet gateway port receiving packets via the system crossbar ingeneral does not change any addressing mode(s), and instead relies onthat the received packet already has been set up with correct addressingfrom the system ingress port that forwarded it.

In the case of link based forwarding domain boundaries, a leaf-subnetgateway port is part of the forwarding domain of the core fabric theparticular system instance represents. Hence, the forwarding mode forpackets arriving on the IB link is the “GRH/ISRN” forwarding mode.

In the case of system crossbar based forwarding domain boundaries, aleaf-subnet gateway port is part of the forwarding domain defined by theremote leaf subnet instance. Hence, the forwarding mode for packetsarriving on the IB link is the “LRH/DLID” forwarding mode.

The output port may selectively represent a core-fabric gateway port oranother leaf-subnet gateway port such as when two leaf subnets areconnected via a single system crossbar, and may also represent a virtualrouter port.

In accordance with the example embodiment, when packets move from theleaf-subnet gateway port via the system crossbar to a core-fabricgateway port, then this transition represents changing from theLRH/DLID-based forwarding mode to the GRH/ISRN-based forwarding mode.

In accordance with the example embodiment, when packets move from acore-fabric gateway port via the system crossbar to the leaf-subnetgateway port, then this transition represents changing from GRH/ISRNaddressing mode to LRH/DLID addressing mode.

In accordance with the example embodiment, when packets move between theleaf-subnet gateway port and another leaf-subnet gateway port (i.e.connected to a different leaf subnet) then the original packet LRH.DLIDvalue is used for looking up the output port, but two alternatives existfor how the LRH is constructed for the next hop as set out below.

As the first alternative, if the packet has no GRH and the destinationport allows “un-modified LID forwarding”, then the packet is forwardedwith the same LRH.DLID. Hence, in this case it is up to managementsoftware to ensure that packet traffic crossing from one leaf-subnetinstance to another without generation of new LRH.DLID is representing aLRH.DLID value that is handled consistently in both leaf subnets sinceby default each leaf subnet instance represents its own LID value space.

As the second alternative, if the packet has GRH that does representISRN based forwarding, then the packet is forwarded if there is an FLSNmatch with the output port. It is to be noted that similar to what waspointed out in the core fabric gateway port discussion above, there isin general no need to perform FLSN matching in this case either as longas the routing and forwarding table setup is done correctly.

Still, in accordance with an embodiment, the use of FLSN checking can beselectively enabled in order to actually verify that the forwarding ofpackets between different forwarding domains are not in conflict withFLSN assignment. In this case also, making the FLSN value check optionalimplies that VM migration between different leaf subnets can beimplemented with more flexibility as set out below regarding VMmigration.

As the further alternative, if the packet has GRH that does notrepresent ISRN based forwarding but the destination port allows“un-modified LID forwarding,” then the packet is forwarded with the sameLRH.DLID. Hence, also in this case, management software ensures thatpacket traffic crossing from one leaf-subnet instance to another withoutgeneration of new LRH.DLID is representing a LRH.DLID value that ishandled consistently in both leaf subnets since by default, each leafsubnet instance represents its own LID value space.

When packets move from the leaf-subnet gateway port to a virtual routerport, then the next step forwarding scheme is a function of the policiesimplemented by the virtual router port. The output port may in generalnot be a physical router port because this implies that a new LRH is tobe constructed in a dynamic fashion based on the packet GRH, and this isperformed via a virtual router port.

Combined Core-Fabric and Leaf-Subnet Virtual-Switch

In accordance with an example embodiment, a combined core-fabric andleaf-subnet virtual-switch construction allows the same set of ports toprovide the properties of both a core-fabric and a leaf-subnetvirtual-switch at the same time. One motivation for this construction isto allow non-GRH based data and control traffic to take place betweennodes directly connected with the one or more system instances in thecore fabric. Forwarding options within the core fabric were describedabove.

From an implementation perspective of the subject embodiment, as long asa physical system port has configuration information that defines therole of other ports that can be used as output ports relative to therole of this port, there is a need to enforce a restriction thatspecific set of ports is defined as a combined Core-Fabric andLeaf-Subnet Virtual-Switch as opposed to allowing partial overlap ofphysical port membership between different virtual switch instances.

Combined Core-Fabric and Leaf-Subnet Virtual-Switch Port

A combined core-fabric and leaf-subnet virtual-switch port in accordancewith an example embodiment supports both the GRH/ISRN forwarding modeand the LRH/DLID forwarding mode.

In order to perform the GRH/ISRN based forwarding, the LRH.DLID is aspecific pre-defined value that is common for all ports in the ISRNbased path. Hence, this LID value can either be a system chip levelconstant, or it preferably can be defined by a per physical portregister.

The combined port can support a configurable number of DLID entries andanother configurable range of ISRN values in terms of logical LinearForwarding Table (LFT) capacity.

It is to be noted that, in general, it is preferred that all ports inthe same virtual switch have the same DLID and/or ISRN LFT capacity sothat the set of values can be supported by any port. However, this ismerely a configuration policy and not an implementation constraint ofthe example embodiment.

Similarly, each virtual switch instance including the combined virtualswitch instance type and the single virtual switch instance type areselectively defined in the example embodiment to logically represent perport Linear Forwarding Tables (LFTs) or a logically common LFT for allthe virtual switch ports. However, this is a firmware option as thephysical LFT implementation is distributed per port or per set of dualports in the example embodiment. One reason for this firmware option isobviously to support the IB Specification compliant IB switch mode, aswell as to be able to use Subnet Manager and routing engines andinitialization engines that do not support distributed LFTs.

Virtual Router Port

A virtual router port is represented in the example embodiment by asystem port number outside the range of physical ports. Hence, thevirtual router port has no directly associated physical link. Anyvirtual switch port can look up a virtual router port, and the virtualrouter port then uses a configurable routing function on the packet GRHin order to define physical output port and next hop LRH.

It is to be noted that the routing function in the example embodimentincludes both hierarchical forwarding between end-points in the samelocal fabric, as well as generic forwarding between independent localfabric instances.

If a packet without a GRH is sent to a virtual router port, thenpreferably the packet is either dropped or a new LRH and physical outputport is generated based on a configurable function.

Each virtual switch port has an associated default virtual router portthat is used as intermediate output port if an LFT based lookup is notpossible because the DLID or ISRN value from the packet is outside theLFT range supported by the port. In this case, the virtual router portis operable to selectively either drop the packet as a default per theIB Specification, or perform a configurable function on the packetheaders in order to define physical output port and next hop LRH.

Physical Router Port

A physical router port is, in the example embodiment, a port thathandles ingress to a local fabric instance or egress from a local fabricinstance. The physical router port is represented as an IB Specificationcompliant router port from the link side.

In the example embodiment, a physical router port is associated with thesame packet forwarding logic as a virtual router port used for packetsarriving from the link side.

In the example, embodiment, a physical router port is looked up orotherwise referenced by a virtual router port or another physical routerport that has performed a selected function on the packet GRH of apacket and then produced a new LRH intended to exit on the link of thephysical router port.

Simple LRH.DLID insertion is provided based on remote DLID valueassociated with remote router port in minimal gateway subnet.

Inter Subnet Route Number (ISRN)

In accordance with the example embodiment, the Inter Subnet Route Number(ISRN) is a value defined by a variable part of a packetGRH.DGID.subnetprefix that is used to index LFT entries within a corefabric instance. The ISRN value space is per core-fabric instance, andthe Fabric Global Manager (FGM) coordinates the use of the ISRN values.

In the example embodiment, packets to be forwarded using the ISRN valuehave a special reserved LRH.DLID value that indicates to the receivingport that the ISRN value should be used for LFT based forwarding.

In an embodiment, an ability for system ports to have “multiplepersonalities” implies that the packet should contain information aboutthe particular type of forwarding mechanism to use as opposed to havingthis (only) as a per port configuration info.

Fabric Local Subnet Number (FLSN)

In accordance with the example embodiment, the Fabric Local SubnetNumber (FLSN) is a value that identifies a destination subnet within alocal fabric. Preferably, all FLSN numbers are allocated and managed bythe Fabric Global Manager (FGM). Any packet that is forwarded based onISRN value also has a destination FLSN value.

An FLSN value is associated with Leaf-Subnet Gateway ports that assume alogical port role, and packets forwarded to such ports will have thepacket FLSN number matched against the port FLSN number in order todetermine that the destination subnet has been reached.

In an embodiment, an ability for system ports to have “multiplepersonalities” implies that the packets are qualified as having reachedtheir destination subnet based on combination of both packet and portinformation as opposed to simply relying upon per port configurationinformation.

Fabric Global Manager (FGM)

In accordance with the example embodiment, the Fabric Global Manager(FGM) is a “super Subnet Manager” that is capable of performingdiscovery and configuration of a complete local fabric instance. The FGMcooperates with “FGM aware” Subnet Managers in each individual leafsubnet.

It is to be noted that each leaf-subnet Subnet Manager is capable ofmanaging its own local leaf-subnet independently of the FGM, but willdepend on the FGM in order to facilitate any inter-subnet traffic thatinvolves any core-fabric or other leaf-subnet in the local fabric.

Identification Subnet Number (ISN)

In accordance with the example embodiment, an Identification SubnetNumber (ISN) is an administratively assigned subnet number that may havea global or a site specific uniqueness. The ISN number is used togetherwith a GUID in order to form GID values that are selectively used forinter-subnet traffic between different local fabric instances.

Hardware GUID (HW GUID)

In accordance with the example embodiment, a hardware GUID is a 64 bitglobally unique identification of a hardware resource. Each physicalsystem instance in the example embodiment has a base HW GUID value. Eachsystem based virtual switch instance has a HW GUID that is derived fromthe base HW GUID in a pre-defined way. Each physical HCA will have abase HW GUID. Each virtual HCA port will have a HW GUID that is derivedfrom the base HW GUID in a pre-defined way.

Virtual GUID (vGUID)

In accordance with the example embodiment, a virtual GUID (vGUID) is a64 bit GUID value that is either globally unique or unique within alocal fabric. A virtual GUID is also guaranteed to be unique relative toany HW GUID present in the local fabric.

A virtual GUID is typically associated with a Virtual Machine instance,and the same vGUID value (one for each vHCA port the VM is using) willnormally be associated with a VM throughout the VM lifecycle that mayinclude multiple migrations between physical servers and vHCA instances.

It is to be noted that a particular vHCA instance is identified by itsHW GUID, but whenever a VM is using it, the vHCA ports can then also beaddressed using the relevant VM vGUID values.

Packet Header Formats and Conventions:

The standards for the packet Local and Global Route Header (LRH/GRH)formats and conventions will now be described. First, the Local RouteHeader (LRH) used in the example embodiment herein is as defined by theInfiniBand (IB) Specification. Similarly, Global Route Header (GRH) usedin the example embodiment herein is as defined by the InfiniBand (IB)Specification, but with a set of two additions/exceptions: 1) theGRH.DGID.SUBNET_PREFIX can have type “ISRN” and 2) theGRH.DGID.SUBNET_PREFIX can have type “LeafSwitchHierarchy.”

When the GRH.DGID.SUBNET_PREFIX is of the type “ISRN,” theGRH.DGID.SUBNET_PREFIX has, in the example embodiment, the followingformat (in addition to the ISRN Type:

-   -   (Destination) Fabric Local Subnet Number    -   Inter Subnet Route Number (i.e. in the core fabric instance        defined by the initial LRH.DLID value in the source leaf subnet)    -   DLID (in destination leaf subnet defined by the Fabric Local        Subnet Number).

When the GRH.DGID.SUBNET_PREFIX is of the type “LeafSwitchHierarchy,”the GRH.DGID.SUBNET_PREFIX has, in the example embodiment, the followingformat (in addition to the LeafSwitchHierarchy Type:

-   -   (Destination) Fabric Local Subnet Number    -   (Destination) Leaf Switch Number

All special sub-fields have run-time defined sizes and offsets. (I.e.enabled by using dedicated TCAM entries to perform the sub-fieldextractions).

Mapping and Selection of SL and VL Values Throughout a Local Fabric:

In accordance with the example embodiment herein, within a singleleaf-subnet, packets with only LRH and no GRH will be using legacy SL toVL mapping. The example embodiment may, selectively as desired ornecessary, extend the local LID and/or SL/VL value space by usingreserved bits and new header version number. Extending the number ofport numbers that can be looked up by the LFT and allow more such“alias” port numbers to represent other SL/VL mappings for an associatedphysical port. (i.e. same effect as the SL/VL handling but consumes LFTmemory rather than relying on proprietary use of reserved LRH bits.)This implies use of multiple DLID values rather than different SL valuesto achieve traffic separation.

Mapping and selection of values within a single leaf-subnet packets withboth LRH and no GRH will by default use, in the example embodiment,legacy SL to VL mapping. The GRH fields are preferably used in the sameway as in core fabrics.

Mapping and selection of values between leaf-subnets and within corefabrics, packets with only LRH are preferably to be handled as in asingle leaf subnet.

Mapping and selection of values between leaf-subnets and within corefabrics, packets with both LRH and GRH will have SL selection based onthe following scheme: each Traffic Class value represents a set ofpossible SL values; a hashing of the Flow Label value selects among theavailable SL values defined by traffic class, and support is providedfor “aggressive” credit wait policy as defined in the “Mapping andSelection of SL and VL values through Gateway Subnets” below.

Mapping and Selection of SL and VL Values Through Gateway Subnets:

By default, the same GRH based schemes as defined for use within a localfabric is supported by the example embodiment. In addition to the basicmapping, an egress router port such as for example a router port sendingpackets to the gateway subnet/router port in the remote fabric, mayclassify all such packets or a subset based on defined criteria ashaving a more “aggressive” credit wait policy where packets will bedropped “early” in order to avoid congestion trees to build up in thelocal fabric.

Mapping and Selection of P Key Values Throughout a Local Fabric:

Each forwarding domain in the example embodiment has a range of P_Keyvalues that can be used for traffic internal to the leaf-subnet withoutany coordination at the local fabric level. That is, the same privaterange can be used by all forwarding domains. Any P_Key value outside thelocal range is allocated via the FGM. The FGM ensures that gateway portsonly accept forwarding of packets with P_Key values that are allowed tobe forwarded into the relevant forwarding domain.

In a further example, the FGM can define special paths that involvesmapping of P_Key values at specific gateway ports such as betweendifferent forwarding domains. In general, such mapping implies that thepacket Invariant CRC (ICRC) is re-computed. As an alternative schemethat involves support from the end nodes such as HCAs, a scheme isprovided where the normal packet ICRC is computed based on a packetP_Key value of 0, but in addition has a reserved bit field in the GRHthat is not included in the ICRC, but which contains a CRC parity valuefor the current P_Key value.

Mapping and Selection of P Key Values Through Gateway Subnets:

The example embodiment as described herein makes, in general, noassumption about coordinated P_Key value allocation between differentlocal fabric instances. Native IB communication is in this case based ona dynamically negotiated P_Key value between the involved local fabricinstances, or based on a P_Key mapping scheme as outlined for the LocalFabric case above.

Forwarding of Multicast Packets Throughout a Local Fabric:

In the example embodiment, each forwarding domain has a range of MLIDvalues that are considered private for the forwarding domain, whereinthe same private range can be used by all forwarding domains. Anymulticast group mapped to a private MLID can only be forwarded withinthe local forwarding domain. The FGM ensures that private MLIDforwarding will not take place across forwarding domain boundaries, andcoordinates the use of a set of shared MLID values that can be used toimplement multicast forwarding throughout the complete local fabricinstance. MGID values to be used via shared MLID values throughout thecomplete local fabric are allocated by the FGM. Such MGID values have aselected bit field that defines it to be an FGM controlled MGID. Thelocal SM within each forwarding domain, is responsible for ensuring thatthere is no conflicting use of FGM controlled MGIDs.

In a further example, virtual router ports are used. In this example,multicast packets with a specific MGID and MLID value in one forwardingdomain can be forwarded with the same MGID value but a different MLIDvalue in another forwarding domain.

Forwarding of Multicast Packets Through Gateway Subnets:

The example embodiment as described herein makes, in general, noassumption about coordinated MGID value allocation between differentlocal fabric instances. Native IB multicast communication with purehardware based forwarding is in this case based on a dynamicallynegotiated MGID value between the involved local fabric instances.

By default in the example embodiment, any multicast forwarding betweendifferent local fabric instances is based on configuring a softwarebased proxy service that operates in association with one or morephysical router ports and receives multicast packets from the localfabric or from the physical router port and forwards the payload in anew multicast packet in the opposite direction. The proxy is,preferably, by default a normal multicast group member on the localfabric side, and has a special relationship with the physical routerport in order to intercept all multicast packets arriving from theremote fabric such as, for example, via the gateway subnet.

In addition to the above, a mechanism is selectively provided to ensurethat multicast packets from the remote fabric are forwarded to the proxyin a way that does not interfere with any multicast forwarding in thelocal fabric. This could use a scheme or a notion of a “default MCdestination” that uses a dedicated MLID in the local fabric. This isviable as long as the receiver(s) are able to receive any number ofMGIDs and forward to the QP(s) associated with the proxy instance(s).This may also include a large number of partitions. If thisfunctionality is hard to ensure for relevant HCAs, then an alternativeembodiment herein is for the system to function to tunnel the IB MCpacket in an IB unicast packet, in a way that is similar to how Enetpackets are tunneled when received on an Enet GW port. The proxyimplementation is then able to interpret and handle the incoming remoteMC packet independently of how it is tunneled on the local fabric.

It is to be noted that, in order to handle any number of partitions,more than one SW proxy may be required in the direction from localfabric to remote fabric. It is to be further noted that for the physicalrouter port, tunneling in both directions is implemented. Also, in thecase where a proxy implementation exists in both fabrics, the tunnelingis implemented by cooperating proxy implementations without the need forany dedicated hardware support. In this connection, a tunneling schemeimplies that the max IB MTU cannot be forwarded as tunneled MC packetssince the original IB headers have to be included as IB payload in thetunneling packet. The proxy is preferably able to handle the case wherethe same MGID value is used for two different logical multicast groupsin different local fabrics.

Initial Discovery and Configuration of a Local Fabric:

In accordance with the example embodiment, by default, each systeminstance operates as a single conventional InfiniBand (IB) switch. Inthis default configuration, any existing IB Subnet Manager or SMP baseddiagnostic and monitoring tool is able to “see” or otherwise learn thecomplete connected fabric as a single IB subnet.

The division of a system instance into multiple virtual switches as wellas defining port types and associated configuration informationrepresents an “out-of-band” configuration policy that is, for example,not part of normal SMA operations. All or most out-of-band configurationoperations can be implemented however via vendor specific SMP operationsas an alternative to “pure” out-of-band configuration. It is to be notedthat since in the example embodiment each system instance has E-netbased management interface that is available independently of any IBlink state, and also supports IPoIB/EoIB, the “out-of-band”configuration does in principle represent IP based access via any of theinterfaces.

It is to be further noted that there is no notion of a pure “local”management interface like PCI-E that is inherently associated with alocal management processor. Still, in order to solve “chicken-and-egg”configuration issues relative to security, the Enet interface mayrepresent an initial source of configuration policy with the assumptionthat the physical system such as for example a specific independentphysical switch chassis, the system of the example embodiment is part ofpreferably has secured the Enet access via port based VLANs or otheraccess control rules. This knowledge is preferably production timedefined “a priori” and is persistent configuration information for thesystem instance.

In configurations where an HCA port representing a chassis internalmanagement processor is present and connected to a specific port on aspecific system instance such as defined by the hardware design, thenthe system instance in accordance with the example embodiment shouldalso have a priori information about the local system port having thisconnectivity.

Based on configuration information supplied initially via the Enetinterface or via the management Host Channel Adapter (HCA), the firmwareof the system of the example embodiment is able to authenticateconfiguration operations from other interfaces and/or other sources onthe management Enet interface as well. Such configuration information isstored reliably and persistently by the firmware of the exampleembodiment.

The firmware in accordance with a further example embodiment may alsohave access to additional “a priori” persistent configurationinformation that allows the firmware to authenticate access from otherinterfaces/sources at initial deployment time. This could include publickeys for vendor-specific management software etc. In addition, privatekeys used for authenticating the type and possible also the specificidentity such as related to the hardware GUID of this vendor instancecould be present as may be necessary or desired.

In accordance with the example embodiment, when port roles and virtualswitches have been configured, the in-band management components such asfor example the Subnet Manager (SM) and FGM instances, are able todiscover their relevant portions of the local physical and/or virtualfabric using both legacy SMP-based subnet discovery methods as well asone or more vendor-specific SMP method(s).

In accordance with the example embodiment, each leaf subnet ispreferably discovered and configured in parallel with other leaf subnetsby independent SM instances.

In the core fabric(s), one or more “master” FGM instances may operateconcurrently with, in general, one master FGM instance per core fabricinstance.

Further, subnet managers in leaf-subnets preferably do not have any SMPaccess (read or write) outside their leaf-subnet, which is outside theirforwarding domain. A legacy subnet manager in a leaf subnet will only beable to discover ordinary (virtual) switch ports and their localconnectivity within the leaf subnet. A fabric-aware subnet manager in aleaf subnet is preferably able to discover gateway ports and theconnectivity they represent, but is not to be able to modify anyconfiguration of the gateway ports. A fabric-aware subnet manager in aleaf subnet will by default preferably not be able to discover anyremote port connectivity beyond a gateway port, nor any further hops,nor be able to modify any configuration information beyond a gatewayport.

In the example embodiment, an FGM instance will by default be able todiscover the complete local fabric. An FGM instance may avoid, by aconfiguration option for the FGM instance for example, or be preventedfrom, by access restrictions in remote virtual systems, performing anyhop-by-hop DR SMP based discovery of the topology outside a localforwarding domain such as outside a specific core fabric instance, oroutside the complete core fabric. Access restrictions may typicallyapply to different leaf subnets that has a different fabric level owner.In this case, the central FGM collects topology information from eachcore-fabric instance as well as topology information from each leafsubnet via the relevant subnet manager or FGM instances and uses thisinformation to evaluate the optimized routing of the complete localfabric.

It is to be noted that, by default, full end-to-end connectivity withina local fabric will exist following a local routing of each forwardingdomain where full connectivity is established between all end-ports ineach leaf subnet, between all end ports and all gateway ports in eachleaf subnet, and between each gateway port in each core fabric instanceand, in particular, between any pair of leaf-subnet gateway ports.

Based on the above full connectivity, the central FGM can selectend-to-end paths for all potential “pair of peers” in the local fabricbased on combining a path in the source leaf subnet, a path in arelevant core fabric instance, if any, and the relevant path in thedestination leaf subnet. The central FGM would need to ensure that anyend-to-end path between the different leaf subnets is not representingany credit loop. However, credit loop avoidance will trivially beensured as long as the following conditions are fulfilled: each leafsubnet represents a fat-tree where gateway ports to core fabricinstances and/or other leaf subnets represents root switch nodes in thelocal leaf subnet, and each core fabric instance is routed with allleaf-subnet gateway ports as leaf nodes/ports within a regular fat-tree.

In accordance with the example embodiment, in order to ensure staticoptimization within the complete local fabric via dedicated or “primary”down-paths, the central FGM selects a path for the target leaf subnetthat will use an egress gateway port in the root switch that representsthe dedicated or “primary” down path for the destination node in thetarget leaf subnet. The information about root switch will in general beavailable from the master Subnet Manager (SM) within the leaf subnet.The master Subnet Manager (SM) in the leaf subnet has selected a rootswitch which may be a virtual switch or a complete physical instance ofthe system of the subject embodiment for the destination node as part ofthe local routing of the leaf subnet.

The selection of gateway port inherently also represents selection of acore fabric instance that currently has connectivity from the sourceleaf subnet and that represents a “reasonable” balancing of inter leafsubnet traffic at this point in time.

The central FGM then selects an ingress gateway port from the sourceleaf subnet to the selected core fabric instance, and an ISRN value thatrepresents the primary path for the selected destination egress gatewayport in the selected core fabric instance.

The selection of ingress gateway port to the core fabric instance alsorepresents a “reasonable” balancing of the inter-leaf subnet traffic atthis point in time.

In accordance with the example embodiment, the path from the source portin the source leaf subnet to the selected ingress gateway port to theselected core fabric instance is defined by the master Subnet Manager(SM) in the source leaf subnet. This path is preferably constructed bythe master Subnet Manager (SM) without any knowledge about potential usefor inter leaf subnet traffic. The central FGM can then construct thecomplete path based on combining the information from the master SubnetManager in both the source and the target leaf subnet with the selectedISRN value for the selected core fabric instance.

In addition to the pure route information defined by the LID and ISRNvalues, the central FGM will also obtain SL values from each involvedmaster Subnet Manager, and then also select Traffic Class and Flow Labelto reflect SL/VL selection within the core fabric and SL selection uponentering of the target leaf subnet.

It is to be noted that in the simplest default case, a single SL valuecan be selected that will be preserved throughout the complete localfabric and with consistent mapping to a fixed VL in all forwardingdomains such as for example in the same way as for a legacy singlesubnet fat-tree topology.

It is to be further noted that, unlike a conventional single subnetfabric where a DLID represents a final destination, and where the sameDLID can be re-routed to use different routes through the fabric as aconsequence of various link and/or switch failures within the fabric,the DLID-ISRN-DLID based end-to-end path within a fabric may need to beupdated as a result of failures as long as there is not a coordinatedre-routing of the DLIDs for ingress gateway ports from the source subnetto the core fabric, as well as the ISRN value representing the egressgateway port from the core fabric instance to the destination leafsubnet. Hence, in the example embodiment, if a gateway port is no longeravailable, then either a re-routing is performed of the relevant DLID orISRN value to represent an alternative gateway port that can provideequivalent forward connectivity, or an update is performed of pathinformation for impacted flows/communication peers.

Also in the example embodiment, in order to perform re-balancing ofoverall inter leaf-subnet paths, the central FGM selectively generatesnew path information for one or more flows and notifies the involvedpeers, or orchestrates an end-to-end re-routing of the currentDLID-ISRN-DLID based path involving all related master subnet Managersand FGM instances.

Maintaining a Global Overview of Physical and Virtual Node Population:

In accordance with the example embodiment, the physical and virtualend-nodes in each leaf subnet are discovered and configured by the localmaster Subnet Manager (SM). Each master Subnet Manager will notify thecentral FGM about its local node population by queries and events. Forcore fabrics with communicating end-nodes including physical and virtualmanagement servers as well as switch management ports, the relevant FGMinstance keeps track of or otherwise maintains information relating tothe population in the same way as the master SMs in the leaf subnets.The central FGM will thereby be able to keep track of the whereabouts ofany end-node based on GUI D, and hence be able to construct the relevantpath information for any required communication.

In general, VM migration within a single leaf-subnet is handled locallywithin the leaf-subnet, but any change of routing or path information iscommunicated to the central FGM instance. VM migration acrossleaf-subnet boundaries will in general be coordinated via the centralFGM.

It is to be noted that the central FGM is in general responsible forcoordination of vGUID and P_Key value ranges across the various leafsubnet and core fabric instance boundaries. Hence, any value allocationwithin any forwarding domain that is not in synchronism with the currentglobal policy will not be allowed to become visible outside the relevantforwarding domain, nor have any negative impact on other forwardingdomains.

Virtual Machine (VM) Migration:

In accordance with the example embodiment, virtual machine (VM)migration within a single leaf subnet can take place without any changeof address information as long as unique DLID values are used to denotethe virtual HCA ports the VM is representing.

Whenever a VM is migrated out of a leaf subnet, then in general, theaddressing information is updated for all peers in the original leafsubnet that have been using LRH only (i.e. no GRH) based addressing toreach the VM, since use of GRH now becomes required. Likewise, whenevera VM is migrated out of a leaf subnet, then the addressing informationis updated for all peers in other forwarding domains currently usingGRH/ISRN based addressing. This is the case as long as it is notpossible to preserve the VMs LID value used in the original subnet inaddition to perform a re-routing of both the DLID for the initialcore-fabric gateway port as well as the ISRN value for the subsequentleaf-subnet gateway port.

In accordance with a further example embodiment, using TCAM basedforwarding and LRH generation via virtual router ports in the systeminstances that directly connects the new leaf subnet a VM is migratedto, it is possible to have an intermediate state where the LID for theinitial core fabric gateway port as well as the ISRN value for thesubsequent leaf subnet gateway port has been re-routed to provideforwarding to the relevant virtual router port, the virtual router portcould do the generation of the final LRH with the new DLID so that thecomplete old addressing information would be valid following themigration.

The above scheme can be combined as necessary or desired with a “lazy”scheme for updating address information for all relevant peers, and bothold (original) and new address information is could then be valid forthe VM concurrently. When it has been confirmed for example by thecentral FGM that no peer no longer depends on the old addressinformation, the old address information can be “recycled”.

This scheme assumes that at least the original ISRN value is allocatedexclusively for the VM (i.e. not for the leaf-subnet gateway port assuch) so that it can be re-routed without causing conflicts for otherdestinations using the same ISRN value for the intermediate core fabricpath.

In the case where VM migration occurs in a way that requires that a newcore fabric instance has to be used to reach the VM after migration,then the same constraint exists for the DLID values used to address thecore fabric gateway port in all communicating leaf subnets.

As special case here is the communication peers inside the original leafsubnet.—As long as these peers have been instructed to use GRH/ISRNbased addressing in the first place, the VM migration can be handledtransparently based on re-routing the original DLID and ISRN values(i.e. initially the original LRH.DLID will have pointed to the VMdirectly, but after migration, this will have been updated to point tothe relevant core-fabric gateway port.

From the perspective of a VM that is being migrated out of a leaf subnetinstance, it will in general have to update address information for allits communication peers since the first hop addressing in particularwill not be the same in different leaf subnets. This is independent ofall the enhancements to achieve address transparency for thecommunication peers outlined in the section above.

It is to be noted that, in all cases above, the assumption is that FLSNchecking is not enabled or that migration only takes place between leafsubnet instances with the same FLSN value, or that the systemimplementation allows some number of FLSN values to be associated witheach port as described above so that a few migrations with conflictingFLSN values can be handled at any point in time, that is until all peershave updated the address information as described above.

Hardware Support for the Leaf-Subnet/Core-Fabric Model:

Within a single leaf subnet, the system of the example embodimentimplements the LRH.DLID based packet forwarding using Linear ForwardingTables (LFTs) according to the InfiniBand (IB) Specification. Bydefault, each virtual switch has an LFT that supports the full 48Kunicast DLID space as well as a multicast forwarding table that supportsthe full 16K multicast DLID space. As long as the POR for the system isto have private unicast and multicast LFTs that is/are shared by pairsof physical ports, the minimum granularity for configuring virtualswitch port membership is the groups of two ports sharing LFTs. Thenwith enhanced SM and FGM routing logic, multiple independent LFTs pervirtual switch may be supported. However, as long as LFTs are shared bymore than one physical port, the dependency is that the actual physicaltopology lends itself to handling of multiple independent LFTs.

It is to be noted that, the sharing of LFTs between physical ports hasimpact for how grouping of ports per cable connector (if more than 4×),as well as how the cabled physical connectivity of a large local fabricshould look like in order for utilization of more than one LFT pervirtual switch to be beneficial. Also, using various proprietary orpre-standard techniques, embodiments of the subject system couldpotentially expand the LRH.DLID unicast space beyond 48K.

In order to identify the boundary between forwarding domains such as forexample between different leaf subnets and/or core fabric instances, thesystem of the example embodiment implements a set of features describedbelow.

First, from within a leaf subnet, any core fabric gateway port isidentified by one or more DLIDs so that forwarding takes place usingnormal LRH.DLID based lookup. In order to identify that a looked upoutput port represents a core fabric gateway port that requires a changein addressing mode from LRH/DLID (based on the original LRH) to GRH/ISRNinformation is provided that allows this change in mode to be determinedbefore the packet is reaching the egress link in the output port. Inorder to avoid packet stalling, this information is made available asearly as possible after the packet has been received at the ingressport, and the new LRH is made ready and well-defined once the arbiterallows the packet to be forwarded to the output port. Hence, theinformation is available along with the lookup of output port in theingress port. Further, in order to allow the same output port to havemore than one “personality” depending on which other local system portis sending the packet to it across the crossbar, having the type ofoutput port available via registers associated with each input port isthe most flexible scheme. This both saves space relative to have theport type encoded in the LFT entry, and it provides flexibility and noneed for access arbitration relative to both shared LFT entries as wellas relative to using a centralized approach where the type of outputport is defined once for the whole chip. The hardware support forconstructing the new packet with the new addressing mode includesgeneration of a fixed LRH.DLID based on an internal register, as well aslogic to identify and extract the correct ISRN bit field from the GRH.

Second, within a core fabric instance, the identification of GRH/ISRNforwarding mode in an ingress port is based on matching the incomingLRH.DLID value with the contents of an internal register. Then, if thepacket LRH.DLID value matches the value contained in the register,forwarding mode is the “GRH/ISRN” forwarding mode of the exampleembodiment. Otherwise, if the packet LRH.DLID value does not match thevalue contained in the register, the forwarding mode is the InfiniBand(IB) legacy “LRH/DLID” forwarding mode.

In the example embodiment, for performance and flexibility, thisregister is preferably replicated for all ports. This implies that it isalso possible to have a leaf subnet virtual switch instance where theSubnet Manager (SM) is not aware of this system feature, but withoutreducing the available LFT size within that leaf subnet. From within acore fabric instance, the route towards a gateway port (or end port) isdefined by the ISRN value that is used to look up the output port fromthe LFTs of each of the “in-route” system ingress ports.

In order to determine that the output port is a gateway to thedestination leaf subnet, the straight forward solution of the exampleembodiment is to compare the packet FLSN number with the FLSN number(s)associated with the looked up output port. However, in order to identifythe required change in addressing mode while still allowing the ingressport to operate in more than one mode, it is sufficient for the ingressport to have a flag per possible output port that tells if the outputport represents a change in addressing mode or that the output port doesnot represent a change in addressing mode. Hence, if the ingressforwarding mode is GRH/ISRN, and the output port is flagged asrepresenting change in addressing mode (i.e. relative to this ingressport), then the addressing mode can deterministically be changed toLRH/DLID based on the DLID value contained in the GRH.DGID.prefix.dlid.It is to be noted that in this case, matching the FLSN value would justrepresent an additional integrity check as long as the ISRN basedrouting within the core fabric instance is correct.

The HW support for constructing the new packet with the new addressingmode includes logic to identify and extract the correct DLID bit fieldfrom the GRH, as well as generation of the new LRH based on theextracted DLID value.

Third, when two leaf subnets are directly connected by different virtualswitches on the same system instance, there is typically a mutualleaf-subnet gateway port relationship between the ports in the twovirtual switches. Hence, from within leaf subnet A, there will be one ormore DLID values that identifies a specific leaf subnet gateway portproviding direct connectivity to leaf subnet B. Then in order todetermine that the output port is a gateway to the destination leafsubnet, the straight forward solution of the example embodiment is tocompare the packet FLSN number with the FLSN number(s) associated withthe looked up output port. However, in order to identify the requiredchange in addressing mode while still allowing the ingress port tooperate in more than one mode, it is sufficient for the ingress port tohave a flag per possible output port that indicates whether the outputport represents a change in addressing mode or whether the output portdoes not represent a change in addressing mode such as for the casesabove.

Still, in the case where the change in addressing mode can be eitherLRH/DLID to GRH/ISRN or LRH/DLID to LRH/DLID (i.e. as in this example),it is not sufficient to have only a single flag value per output port.Hence, the per output port flags preferably identify the type of gatewayport, and not just change of addressing mode. Again, however, using FLSNmatching is an alternative, but this is still primarily an integritycheck as long as the forwarding between leaf subnet instances iscorrectly implemented.

Additional Hardware Support for the Leaf-Subnet/Core-Fabric Model:

In order to support an arbitrary number of incoming DGID.subnetprefixvalues but still allow DGID.GUID values to be verified for a targetphysical or virtual HCA port, such target (v)HCAs in accordance with anembodiment include a mode wherein checking incoming GRH.DGID against thehardware and alias GID values for a physical or virtual HCA port isrestricted to the GUID portion only, and wherein the subnet prefixportion is ignored.

This mode may be selectively specified individually for each GID tableentry. However, from a HW implementation perspective, it may bedifficult to have an associative match against several table entries andthen also have multiple matches based on different matching criteria.Hence, a mode where the check is always done for only the GUID part issufficient in accordance with an example embodiment.

The assumption is then that each virtual HCA port is set up with two ormore GID table entries that are unique also when only the GUID part isconsidered. The normal or default case is that the relevant hwGUID ispresent along with a vGUID that represents the currently active VM. Thetwo corresponding GID table entries will each have a subnet prefix valuebased on the assigned ISN value.

Method for Using Subnet Prefix Values in (GRH) for (LFT) Lookup

FIG. 16 illustrates a method 1600 in accordance with an embodiment forusing packet headers for linear forwarding table lookup in a networkswitch environment in a high performance computing environment. Themethod of the figure will now be described. First and second subnets areprovided at a computer environment including a network fabric. The firstsubnet is different than the second subnet. The first subnet comprises afirst set of network switches wherein each of the first set of networkswitches includes a linear forwarding table (LFT) and a plurality ofphysical ports. The second subnet comprises a second set of networkswitches wherein each of the second set of network switches includes anLFT and a plurality of physical ports. This is the environment of theexample embodiment of the method. Overall, the method 1600 allows a samefirst LFT of a first network switch of the first set of network switchesto be used for both intra-subnet forwarding decisions of packets to beforwarded between ones of the first set of network switches of the firstsubnet, and inter-subnet forwarding decisions of the packets to beforwarded between the first and second sets of network switches of thefirst and second subnets, respectively.

In a first step 1610, a first packet is received at the first networkswitch of the first set of network switches of the first subnet. A firstportion of a header of the first packet is inspected at step 1620. Afirst condition of the first portion of the header of the first packetis determined at step 1630, and a second condition of the first portionof the header of the first packet is determined at step 1640.

The intra-subnet forwarding decision is selectively made at step 1630when the first portion of the header of the first packet is determinedto have or otherwise exhibit the first condition. Otherwise, when thefirst portion of the header of the first packet is determined to nothave or otherwise exhibit the first condition, the method continues tothe inter-subnet forwarding decision step 1640.

When the intra-subnet forwarding decision is selectively made at step1630 such as when the first portion of the header of the first packet isdetermined to have or otherwise exhibit the first condition, the packetis handled at step 1635 using intra-subnet (LRH/DLID) forwarding inaccordance with an example embodiment.

Similarly, when the inter-subnet forwarding decision is selectively madeat step 1640 such as when the first portion of the header of the firstpacket is determined to have or otherwise exhibit the second condition,the packet is handled at step 1645 using inter-subnet (GRH/ISRN)forwarding in accordance with an example embodiment.

At step 1645, the inter-subnet forwarding is selectively made using thefirst LFT of the first network switch in accordance with a secondcondition of the first portion of the header of the first packet forrouting the first packet to a selected one of the set of second networkswitches of the second subnet.

In the example embodiment, the inspecting the first portion of theheader of the first packet comprises inspecting a local route header(LRH) portion of the header of the first packet. Further in accordancewith the example embodiment, the determining the condition of the firstportion of the header of the first packet comprises comparing a selectedportion of the LRH relative to a predetermined value stored at the firstnetwork switch, and determining the first condition in accordance with amis-match between the selected portion of the LRH and the predeterminedvalue, or determining the second condition in accordance with a matchbetween the selected portion of the LRH and the predetermined value.

Further in the example embodiment, the determining the second conditionin accordance with a match between the selected portion of the LRH andthe predetermined value includes a step of using the LFT to index aSpecial Switch Port (SSP) using the selected portion of the LRH.

In particular with regard to the example embodiment, the routing of thefirst packet to a selected one of the set of second network switches ofthe second subnet includes selecting a section of a global route header(GRH) portion of the header of the first packet, the selected section ofthe GRH portion of the header storing an Inter-Subnet-Route-Number(ISRN), using the ISRN of the GRH to index the first LFT of the firstnetwork switch, and routing the first packet to the selected one of theset of second network switches of the second subnet in accordance withthe indexing the first LFT using the ISRN of the GRH, wherein the secondsubnet is an intermediate core fabric of the network switch environment.

Further in accordance with the example embodiment, when neither thefirst nor second conditions of the first portion of the header of thefirst packet are determined at steps 1630 and 1640, the packet isdiscarded at step 1650. More particularly, in the example embodiment,the packet is discarded at step 1650 since none of the forwardingconditions are met.

Subnet Number Aliasing

In accordance with an embodiment, as described above, various leafsubnets and core fabrics, as well as other logical portions of a networkswitched topology, can be associated with at least one “Fabric LocalSubnet Number” (FLSN). A global fabric manager (a manager that overseesand has the authority to configure the various leaf subnets and corefabrics) can, on certain occasions, reconfigure a topology in such a waythat routing to or from end points in a topology can become confused.

In accordance with an embodiment, for example, a global fabric managercan determine, for one or a variety of reasons, that an original leafsubnet comprising a number of switches should be divided into two ormore new leaf subnets. However, packets routed to the original leafsubnet can carry, in the designation field, a FLSN of the original leafsubnet. While it is possible that at least one of new leaf subnets splitoff from the original leaf subnet can continue to maintain this FLSN ofthe original leaf subnet, other leaf subnets that split from theoriginal leaf subnet can be assigned a different FLSN. This presents aproblem in that, until the entirety of the fabric is updated with theFLSN of the one or more new leaf subnets assigned a new FLSN, packetscannot be routed to their correct destination.

In accordance with an embodiment, subnet number aliasing provides for atemporary assignment of a duplicate FLSN until the remainder of thefabric has been updated to reflect a change in FLSN of a split leafsubnet.

FIG. 17 shows an exemplary system for supporting subnet number aliasing,in accordance with an embodiment.

In accordance with an embodiment, a network fabric 1700 can comprise anumber of subnets/sub-fabrics, such as core fabric 1701, core fabric1702, leaf subnet 1703, and leaf subnet 1704. Each of thesubnets/sub-fabrics can comprise a number of switches, nodes . . . etc.In addition, each of the subnets/sub-fabrics can be assigned, by aglobal fabric manager 1720, a fabric local subnet number (describedabove), such as FLSN A 1711 assigned to core fabric 1701, FLSN Bassigned to core fabric 1702, FLSN C 1713 assigned to leaf subnet 1703,and FLSN D 1714 assigned to leaf subnet 1704.

In accordance with an embodiment, and as described above, packetstraveling within the network fabric 1700 can comprise a FLSN within theroute header of the packet (i.e., as part of the global route header).The FLSN can be used to determine a correct destinationsubnet/sub-fabric.

FIG. 18 shows an exemplary system for supporting subnet number aliasing,in accordance with an embodiment.

In accordance with an embodiment, a network fabric 1700 can comprise anumber of subnets/sub-fabrics, such as core fabric 1701, core fabric1702, leaf subnet 1703, and leaf subnet 1704. Each of thesubnets/sub-fabrics can comprise a number of switches, nodes . . . etc.In addition, each of the subnets/sub-fabrics can be assigned, by aglobal fabric manager 1720, a fabric local subnet number (describedabove), such as FLSN A 1711 assigned to core fabric 1701, FLSN Bassigned to core fabric 1702, FLSN C 1713 assigned to leaf subnet 1703,and FLSN D 1714 assigned to leaf subnet 1704.

In accordance with an embodiment, the global fabric manager candetermine that leaf subnet 1703 should be split into multiple leafsubnets. In the embodiment shown in the figure, the global fabricmanager has determined that leaf subnet 1703 should be split into twosubnets, the original leaf subnet 1703 and new leaf subnet 1801, whichhas been assigned FLSN E 1811 by the global fabric manager.

In accordance with an embodiment, in order to keep any traffic directedto nodes that were originally in leaf subnet 1703, but are now containedwithin split off leaf subnet 1801, the global fabric manager can assignan alias FLSN C 1813 to leaf subnet 1801 for a period of time.

In accordance with an embodiment, the period of time for which aliasFLSN C 1813 is assigned by the global fabric manager to leaf subnet 1801can be preset, configurable, or indefinite. As an example, alias FLSN C1813 can assigned by the global fabric manager to leaf subnet 1801 for aconfigurable period of time, wherein the period is an amount of timerequired for the remainder of the fabric 1700, and any other connectedfabrics and/or nodes to update their routing tables in accordance withthe addition of newly split off leaf subnet 1801. This time may not beknown in advance, and the global fabric manager may therefore receiveconfirmation from all relevant parties that the new address informationis received and handled before removing the alias FLSN 1813.

FIG. 19 shows an exemplary system for supporting subnet number aliasing,in accordance with an embodiment.

In accordance with an embodiment, a network fabric 1700 can comprise anumber of subnets/sub-fabrics, such as core fabric 1701, core fabric1702, and combined leaf subnet 1901. Each of the subnets/sub-fabrics cancomprise a number of switches, nodes . . . etc. In addition, each of thesubnets/sub-fabrics can be assigned, by a global fabric manager 1720, afabric local subnet number (described above), such as FLSN A 1711assigned to core fabric 1701, FLSN B assigned to core fabric 1702, FLSNC 1913 and FLSN D 1914 assigned to combined leaf subnet 1901.

In accordance with an embodiment, the global fabric manager candetermine two or more subnets (e.g., leaf subnets) should be merged intoa smaller number of subnets. In the example depicted in FIG. 19, theglobal fabric manager can determine that previous (see FIG. 17) leafsubnets 1703 and 1704 should be merged into a combined leaf subnet 1901.In the embodiment shown in the figure, the global fabric manager hasassigned FLSNC C 1913 to the combined leaf subnet 1901.

In accordance with an embodiment, in order to keep any traffic directedto nodes that were originally in leaf subnet 1704, but are now containedwithin combined leaf subnet 1901, the global fabric manager can assignan alias FLSN D 1914 to the combined leaf subnet 1901 for a period oftime.

In accordance with an embodiment, the period of time for which aliasFLSN D 1914 is assigned by the global fabric manager to combined leafsubnet 1901 can be preset, configurable, or indefinite. As an example,alias FLSN D 1914 can assigned by the global fabric manager to combinedleaf subnet 1901 for a configurable period of time, wherein the periodis an amount of time required for the remainder of the fabric 1700, andany other connected fabrics and/or nodes to update their routing tablesin accordance with the consolidation of original leaf subnets 1703 and1704 into combined leaf subnet 1901. This time may not be known inadvance, and the global fabric manager may therefore receiveconfirmation from all relevant parties that the new address informationis received and handled before removing the alias FLSN 1914.

FIG. 20 shows an exemplary system for supporting subnet number aliasing,in accordance with an embodiment.

In accordance with an embodiment, a network fabric 1700 can comprise anumber of subnets/sub-fabrics, such as core fabric 1701, core fabric1702, leaf subnet 1703, and leaf subnet 1704. Each of thesubnets/sub-fabrics can comprise a number of switches, nodes . . . etc.In addition, each of the subnets/sub-fabrics can be assigned, by aglobal fabric manager 1720, a fabric local subnet number (describedabove), such as FLSN A 1711 assigned to core fabric 1701, FLSN Bassigned to core fabric 1702, FLSN C 1713 assigned to leaf subnet 1703,and FLSN D 1714 assigned to leaf subnet 1704.

In accordance with an embodiment, a virtual machine R 2001, from subnetR (not shown) can be migrated to leaf subnet 1703. On the migration, inorder to keep packets addressed to VM R routed corrected, the globalfabric manager can assign an alias FLSN R 2002 to the leaf subnet 1703for a period of time (FLSN R being the fabric local subnet number of thesubnet from which VM R migrated from, namely subnet R).

In accordance with an embodiment, the period of time for which aliasFLSN R 2002 is assigned by the global fabric manager to leaf subnet 1703can be preset, configurable, or indefinite. As an example, alias FLSN R2002 can assigned by the global fabric manager to combined leaf subnet1703 for a configurable period of time, wherein the period is an amountof time required for the remainder of the fabric 1700, and any otherconnected fabrics and/or nodes to update their routing tables inaccordance with the migration of VM R 2001 to leaf subnet 1703. Thistime may not be known in advance, and the global fabric manager maytherefore receive confirmation from all relevant parties that the newaddress information is received and handled before removing the aliasFLSN 2002.

FIG. 21 is a flow chart of a method for supporting subnet numberaliasing in a high performance computing environment, in accordance withan embodiment.

At step 2100, the method can provide, at one or more computers,including one or more microprocessors a network fabric, the networkfabric comprising a first subnet, the first subnet comprising a firstset of network switches, a second subnet, the second subnet comprising asecond set of network switches, and a global fabric manager, the globalfabric manager having administrative control within the first subnet andsecond subnet.

At step 2120, the method can assign, by the global fabric manager, afirst fabric local subnet number (FLSN) to the first subnet and a secondFLSN to the second subnet, wherein the first FLSN and the second FLSNare different.

At step 2130, the method can monitor, by the global fabric manager, thenetwork fabric.

At step 2140, the method can, based upon the monitoring, reconfigure thenetwork fabric.

At step 2150, the method can, upon reconfiguring the network fabric,assign at least one alias FLSN for a period of time.

Features of the example embodiments can be implemented in, using, orwith the assistance of a computer program product which is a storagemedium (media) or computer readable medium (media) having instructionsstored thereon/in which can be used to program a processing system toperform any of the features presented herein. The storage medium caninclude, but is not limited to, any type of disk including floppy disks,optical discs, DVD, CD-ROMs, microdrive, and magneto-optical disks,ROMs, RAMs, EPROMs, EEPROMs, DRAMs, VRAMs, flash memory devices,magnetic or optical cards, nanosystems (including molecular memory ICs),or any type of media or device suitable for storing instructions and/ordata.

Stored on any one of the machine readable medium (media), features ofthe example embodiments can be incorporated in software and/or firmwarefor controlling the hardware of a processing system, and for enabling aprocessing system to interact with other mechanism utilizing the resultsof the example embodiments. Such software or firmware may include, butis not limited to, application code, device drivers, operating systemsand execution environments/containers.

Features of the example embodiment may also be implemented in hardwareusing, for example, hardware components such as application specificintegrated circuits (ASICs). Implementation of the hardware statemachine so as to perform the functions described herein will be apparentto persons skilled in the relevant art.

Additionally, example embodiments may be conveniently implemented usingone or more conventional general purpose or specialized digitalcomputer, computing device, machine, or microprocessor, including one ormore processors, memory and/or computer readable storage mediaprogrammed according to the teachings of the present disclosure.Appropriate software coding can readily be prepared by skilledprogrammers based on the teachings of the present disclosure, as will beapparent to those skilled in the software art.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example, and notlimitation. It will be apparent to persons skilled in the relevant artthat various changes in form and detail can be made therein withoutdeparting from the spirit and scope of the invention.

The example embodiments have been described above with the aid offunctional building blocks illustrating the performance of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have often been arbitrarily defined herein for theconvenience of the description. Alternate boundaries can be defined solong as the specified functions and relationships thereof areappropriately performed. Any such alternate boundaries are thus withinthe scope and spirit of the claimed invention.

The foregoing description of example embodiments have been provided forthe purposes of illustration and description. It is not intended to beexhaustive or to limit the invention to the precise forms disclosed. Thebreadth and scope of the example embodiments should not be limited byany of the above-described exemplary embodiments. Many modifications andvariations will be apparent to the practitioner skilled in the art. Themodifications and variations include any relevant combination of thedisclosed features. The embodiments were chosen and described in orderto best explain the principles of the claimed invention and itspractical application, thereby enabling others skilled in the art tounderstand the claimed invention for various embodiments and withvarious modifications that are suited to the particular usecontemplated. It is intended that the scope of the claimed invention bedefined by the following claims and their equivalents.

What is claimed is:
 1. A method for supporting subnet number aliasing in a high performance computing environment, comprising: providing, at one or more computers, including one or more microprocessors a network fabric, the network fabric comprising a first subnet, the first subnet comprising a first set of network switches; a second subnet, the second subnet comprising a second set of network switches; and a global fabric manager, the global fabric manager having administrative control within the first subnet and second subnet; assigning, by the global fabric manager, a first fabric local subnet number (FLSN) to the first subnet and a second FLSN to the second subnet, wherein the first FLSN and the second FLSN are different; monitoring, by the global fabric manager, the network fabric; based upon the monitoring, reconfiguring the network fabric; and upon reconfiguring the network fabric, assigning at least one alias FLSN for a period of time.
 2. The method of claim 1, wherein reconfiguring the network fabric comprises merging the second subnet into the first subnet; and wherein assigning at least one alias FLSN for a period of time comprises assigning an alias second FLSN to the first subnet for a period of time.
 3. The method of claim 2, wherein the period of time is determined by the global fabric manager, the determination being based upon a route recalculation time of the network fabric based upon the merger of the second subnet into the first subnet.
 4. The method of claim 1, wherein reconfiguring the network fabric comprises splitting the first subnet into the first subnet and a third subnet, wherein a third FLSN is assigned by the global fabric manager to the third subnet; and wherein assigning at least one alias FLSN for a period of time comprises assigning an alias first FLSN to the third subnet for a period of time.
 5. The method of claim 4, wherein the period of time is determined by the global fabric manager, the determination being based upon a route recalculation time of the network fabric based upon the splitting of the first subnet into the first subnet and the third subnet.
 6. The method of claim 1, wherein reconfiguring the network fabric comprises migrating a virtual machine, running on the second subnet, to the first subnet; and wherein assigning at least one alias FLSN for a period of time comprises assigning an alias second FLSN to the first subnet for a period of time.
 7. The method of claim 6, wherein the period of time is determined by the global fabric manager, the determination being based upon a route recalculation time of the network fabric based upon the migration of the virtual machine from the second subnet to the first subnet.
 8. A system for supporting subnet number aliasing in a high performance computing environment, the system comprising: a computer environment comprising: a network fabric, the network fabric comprising a first subnet, the first subnet comprising a first set of network switches; a second subnet, the second subnet comprising a second set of network switches; and a global fabric manager, the global fabric manager having administrative control within the first subnet and second subnet; wherein the global fabric manager assigns a first fabric local subnet number (FLSN) to the first subnet and a second FLSN to the second subnet, wherein the first FLSN and the second FLSN are different; wherein the global fabric manager monitors the network fabric; wherein, based upon the monitoring, the global fabric manager reconfigures the network fabric; and wherein upon reconfiguring the network fabric, the global fabric manager assigns at least one alias FLSN for a period of time.
 9. The system of claim 8, wherein reconfiguring the network fabric comprises merging the second subnet into the first subnet; and wherein assigning at least one alias FLSN for a period of time comprises assigning an alias second FLSN to the first subnet for a period of time.
 10. The system of claim 9, wherein the period of time is determined by the global fabric manager, the determination being based upon a route recalculation time of the network fabric based upon the merger of the second subnet into the first subnet.
 11. The system of claim 8, wherein reconfiguring the network fabric comprises splitting the first subnet into the first subnet and a third subnet, wherein a third FLSN is assigned by the global fabric manager to the third subnet; and wherein assigning at least one alias FLSN for a period of time comprises assigning an alias first FLSN to the third subnet for a period of time.
 12. The system of claim 11, wherein the period of time is determined by the global fabric manager, the determination being based upon a route recalculation time of the network fabric based upon the splitting of the first subnet into the first subnet and the third subnet.
 13. The system of 8, wherein reconfiguring the network fabric comprises migrating a virtual machine, running on the second subnet, to the first subnet; and wherein assigning at least one alias FLSN for a period of time comprises assigning an alias second FLSN to the first subnet for a period of time.
 14. The system of claim 13, wherein the period of time is determined by the global fabric manager, the determination being based upon a route recalculation time of the network fabric based upon the migration of the virtual machine from the second subnet to the first subnet.
 15. A non-transitory machine readable medium, including instructions stored thereon for supporting subnet number aliasing in a high performance computing environment, which when read and executed by one or more computers caused the one or more computers to perform steps comprising: providing, at one or more computers, including one or more microprocessors a network fabric, the network fabric comprising a first subnet, the first subnet comprising a first set of network switches; a second subnet, the second subnet comprising a second set of network switches; and a global fabric manager, the global fabric manager having administrative control within the first subnet and second subnet; assigning, by the global fabric manager, a first fabric local subnet number (FLSN) to the first subnet and a second FLSN to the second subnet, wherein the first FLSN and the second FLSN are different; monitoring, by the global fabric manager, the network fabric; based upon the monitoring, reconfiguring the network fabric; and upon reconfiguring the network fabric, assigning at least one alias FLSN for a period of time.
 16. The non-transitory machine readable medium of claim 15, wherein reconfiguring the network fabric comprises merging the second subnet into the first subnet; and wherein assigning at least one alias FLSN for a period of time comprises assigning an alias second FLSN to the first subnet for a period of time.
 17. The non-transitory machine readable medium of claim 16, wherein the period of time is determined by the global fabric manager, the determination being based upon a route recalculation time of the network fabric based upon the merger of the second subnet into the first subnet.
 18. The non-transitory machine readable medium of claim 15, wherein reconfiguring the network fabric comprises splitting the first subnet into the first subnet and a third subnet, wherein a third FLSN is assigned by the global fabric manager to the third subnet; and wherein assigning at least one alias FLSN for a period of time comprises assigning an alias first FLSN to the third subnet for a period of time.
 19. The non-transitory machine readable medium of claim 18, wherein the period of time is determined by the global fabric manager, the determination being based upon a route recalculation time of the network fabric based upon the splitting of the first subnet into the first subnet and the third subnet.
 20. The non-transitory machine readable medium of claim 15, wherein reconfiguring the network fabric comprises migrating a virtual machine, running on the second subnet, to the first subnet; and wherein assigning at least one alias FLSN for a period of time comprises assigning an alias second FLSN to the first subnet for a period of time 